Sounding procedure including uplink multiple-user transmission in a  high efficiency wireless lan

ABSTRACT

The present disclosure relates to a sounding procedure including uplink multi-user transmission in a High Efficiency WLAN (HEW). According to one aspect of the present disclosure, a method for transmitting a response frame by a responding Station (STA) to a transmitting STA in a wireless local area network may be provided. The method may include receiving, from the transmitting STA, a trigger frame including information related to a Modulation and Coding Scheme (MCS) for the response frame, and transmitting, to the transmitting STA, the response frame according to an MCS determined based on the information related to the MCS for the response frame included in the trigger frame. When a type of the response frame corresponds to a Multiple-User (MU) type, a same MCS may be applied to the response frame by a plurality of STAs including the STA and at least one other STA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional Application No.62/081,484, filed on Nov. 18, 2014, and U.S. Provisional Application No.62/170,072, filed on Jun. 2, 2015, which are hereby incorporated byreference as if fully set forth herein.

BACKGROUND

1. Technical Field

The present disclosure relates to a Wireless Local Area Network (WLAN),and more particularly, to a method, apparatus, and software forperforming a sounding procedure including uplink multi-user transmissionin a High Efficiency WLAN (HEW), and a recording medium that stores thesoftware.

2. Related Art

Along with the recent development of information and telecommunicationtechnology, various wireless communication techniques have beendeveloped. Among them, the WLAN enables a user to wirelessly access theInternet based on radio frequency technology in a home, an office, or aspecific service area using a portable terminal such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), a smartphone, etc.

To overcome limitations in communication speed that the WLAN faces, therecent technical standards have introduced a system that increases thespeed, reliability, and coverage of a wireless network. For example, theInstitute of Electrical and Electronics Engineers (IEEE) 802.11nstandard has introduced Multiple Input Multiple Output (MIMO) that isimplemented using multiple antennas at both a transmitter and a receiverin order to support High Throughput (HT) at a data processing rate of upto 540 Mbps, minimize transmission errors, and optimize data rates.

In recent times, to support increased numbers of devices supportingWLAN, such as smartphones, more Access Points (APs) have been deployed.Despite increase in use of WLAN devices supporting the Institute ofElectrical and Electronics Engineers (IEEE) 802.11ac standard, thatprovide high performance relative to WLAN devices supporting the legacyIEEE 802.11g/n standard, a WLAN system supporting higher performance isrequired due to WLAN users' increased use of high volume content such asa ultra high definition video. Although a conventional WLAN system hasaimed at increase of bandwidth and improvement of a peak transmissionrate, actual users thereof could not feel drastic increase of suchperformance.

In a task group called IEEE 802.11ax, High Efficiency WLAN (HEW)standardization is under discussion. The HEW aims at improvingperformance felt by users demanding high-capacity, high-rate serviceswhile supporting simultaneous access of numerous stations in anenvironment in which a plurality of APs is densely deployed and coverageareas of APs overlap.

However, there is no specified method for performing a soundingprocedure including uplink multi-user transmission in a HEW.

SUMMARY

Objects of the present disclosure is to provide a method and apparatusfor performing a sounding procedure including uplink multi-usertransmission in a High Efficiency WLAN (Wireless Local Area Network)(HEW).

The objects of the present disclosure are not limited to the foregoingdescriptions, and additional objects will become apparent to thosehaving ordinary skill in the pertinent art to the present disclosurebased upon the following descriptions.

In an aspect of the present disclosure, a method for transmitting aresponse frame by a responding Station (STA) to a transmitting STA in awireless local area network may be provided. The method may includereceiving, from the transmitting STA, a trigger frame includinginformation related to a Modulation and Coding Scheme (MCS) for theresponse frame, and transmitting, to the transmitting STA, the responseframe according to an MCS determined based on the information related tothe MCS for the response frame included in the trigger frame. When atype of the response frame corresponds to a Multiple-User (MU) type, asame MCS may be applied to the response frame by a plurality of STAsincluding the STA and at least one other STA.

In another aspect of the present disclosure, a method for receiving aresponse frame by a transmitting STA from a responding STA in a wirelesslocal area network may be provided. The method may include transmitting,to the responding STA, a trigger frame including information related toan MCS for the response frame, and receiving, from the responding STA,the response frame according to an MCS determined based on theinformation related to the MCS for the response frame included in thetrigger frame. When a type of the response frame corresponds to an MUtype, a same MCS may be applied to the response frame by a plurality ofSTAs including the STA and at least one other STA.

In another aspect of the present disclosure, a responding STA apparatusfor transmitting a response frame to a transmitting STA in a wirelesslocal area network may be provided. The responding STA apparatus mayinclude a baseband processor, a Radio Frequency (RF) transceiver, amemory, etc. The baseband processor may be configured to receive, fromthe transmitting STA, a trigger frame including information related toan MCS for the response frame using the transceiver, and to transmit, tothe transmitting STA, the response frame using the transceiver accordingto an MCS determined based on the information related to the MCS for theresponse frame included in the trigger frame. When a type of theresponse frame corresponds to an MU type, a same MCS may be applied tothe response frame by a plurality of STAs including the STA and at leastone other STA.

In another aspect of the present disclosure, a responding STA apparatusfor receiving a response frame by a transmitting STA in a wireless localarea network may be provided. The responding STA apparatus may include abaseband processor, an RF transceiver, a memory, etc. The basebandprocessor may be configured to transmit, to the responding STA, atrigger frame including information related to an MCS for the responseframe using the transceiver, and to receive, from the responding STA,the response frame using the transceiver according to an MCS determinedbased on the information related to the MCS for the response frameincluded in the trigger frame. When a type of the response framecorresponds to an MU type, a same MCS may be applied to the responseframe by a plurality of STAs including the STA and at least one otherSTA.

In another aspect of the present disclosure, a software orcomputer-readable medium having instructions executable for a respondingSTA to transmit a response frame to a transmitting STA in a wirelesslocal area network may be provided. The executable instructions maycause the responding STA to receive, from the transmitting STA, atrigger frame including information related to an MCS for the responseframe, and to transmit, to the transmitting STA, the response frameaccording to an MCS determined based on the information related to theMCS for the response frame included in the trigger frame. When a type ofthe response frame corresponds to an MU type, a same MCS may be appliedto the response frame by a plurality of STAs including the STA and atleast one other STA.

In another aspect of the present disclosure, a software orcomputer-readable medium having instructions executable for a respondingSTA to receive a response frame by a transmitting STA in a wirelesslocal area network may be provided. The executable instructions maycause the responding STA to transmit, to the responding STA, a triggerframe including information related to an MCS for the response frame,and to receive, from the responding STA, the response frame according toan MCS determined based on the information related to the MCS for theresponse frame included in the trigger frame. When a type of theresponse frame corresponds to an MU type, a same MCS may be applied tothe response frame by a plurality of STAs including the STA and at leastone other STA.

It is to be understood that both the foregoing summarized features areexemplary aspects of the following detailed description of the presentdisclosure without limiting the scope of the present disclosure.

According to the present disclosure, a method and apparatus forperforming a sounding procedure including uplink multi-user transmissionin a High Efficiency WLAN (Wireless Local Area Network) (HEW) can beprovided.

The advantages of the present disclosure are not limited to theforegoing descriptions, and additional advantages will become apparentto those having ordinary skill in the pertinent art to the presentdisclosure based upon the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the disclosure andtogether with the description serve to explain the principle of thedisclosure. In the drawings:

FIG. 1 is a block diagram of a Wireless Local Area Network (WLAN)device;

FIG. 2 is a schematic block diagram of an exemplary transmitting signalprocessing unit in a WLAN;

FIG. 3 is a schematic block diagram of an exemplary receiving signalprocessing unit in a WLAN;

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs);

FIG. 5 is a conceptual diagram illustrating a procedure for transmittinga frame in Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) for avoiding collisions between frames in a channel;

FIG. 6 depicts an exemplary frame structure in a WLAN system;

FIG. 7 depicts an exemplary HE PPDU frame format.

FIG. 8 depicts an exemplary High Efficiency (HE) Physical layer ProtocolData Unit (PPDU) frame format according to the present disclosure;

FIG. 9 depicts subchannel allocation in a HE PPDU frame format accordingto the present disclosure;

FIG. 10 depicts a subchannel allocation method according to the presentdisclosure;

FIG. 11 depicts the starting and ending points of an High EfficiencyLong Training Field (HE-LTF) field in a HE PPDU frame format accordingto the present disclosure;

FIG. 12 depicts a High Efficiency SIGnal B (HE-SIG-B) field and a HighEfficiency SIGnal C (HE-SIG-C) field in the HE PPDU frame formataccording to the present disclosure;

FIG. 13 depicts another example of a HE PPDU frame format according tothe present disclosure;

FIG. 14 depicts an exemplary HE PPDU frame format for a wide channelband according to the present disclosure;

FIG. 15 depicts another exemplary HE PPDU frame format according to thepresent disclosure;

FIGS. 16 and 17 depict exemplary HE PPDU frame formats supportingbeamforming according to the present disclosure;

FIG. 18 depicts an exemplary sounding protocol according to the presentdisclosure;

FIG. 19 depicts an exemplary NDPA frame format according to the presentdisclosure;

FIG. 20 depicts another exemplary sounding protocols according to thepresent disclosure;

FIG. 21 depicts yet another exemplary sounding protocols according tothe present disclosure;

FIG. 22 depicts a sounding procedure including Uplink (UL) Single User(SU) feedbacks according to the present disclosure;

FIG. 23 depicts a sounding procedure including a UL Multi-User (MU)feedback according to the present disclosure;

FIGS. 24 and 25 depict UL SU transmission of a feedback frame and UL MUtransmission of feedback frames in a sounding procedure according to thepresent disclosure;

FIG. 26 depicts an exemplary format of a Beamforming Report Poll frameaccording to the present disclosure;

FIG. 27 depicts an exemplary method according to the present disclosure;and

FIG. 28 depicts another exemplary method according to the presentdisclosure.

DETAILED DESCRIPTION

In the following detailed description, only certain embodiments of thepresent disclosure have been shown and described, simply by way ofillustration. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present disclosure.Accordingly, the drawings and description are to be regarded asillustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

In a Wireless Local Area network (WLAN), a Basic Service Set (BSS)includes a plurality of WLAN devices. A WLAN device may include a MediumAccess Control (MAC) layer and a PHYsical (PHY) layer according toInstitute of Electrical and Electronics Engineers (IEEE) 802.11 seriesstandards. In the plurality of WLAN devices, at least one the WLANdevice may be an Access Point (AP) and the other WLAN devices may benon-AP Stations (non-AP STAs). Alternatively, all of the plurality ofWLAN devices may be non-AP STAs in an ad-hoc networking environment. Ingeneral, AP STA and non-AP STA may be each referred to as a STA or maybe collectively referred to as STAs. However, for ease of descriptionherein, only the non-AP STAs may be referred to herein as the STAs.

FIG. 1 is a block diagram of a WLAN device.

Referring to FIG. 1, a WLAN device 1 includes a baseband processor 10, aRadio Frequency (RF) transceiver 20, an antenna unit 30, a memory 40,which may be or may include a non-transitory computer-readable medium,an input interface unit 50, an output interface unit 60, and a bus 70.

The baseband processor 10 may be simply referred to as a processor,performs baseband signal processing described in the presentspecification, and includes a MAC processor (or MAC entity) 11 and a PHYprocessor (or PHY entity) 15.

In an embodiment of the present disclosure, the MAC processor 11 mayinclude a MAC software processing unit 12 and a MAC hardware processingunit 13. The memory 40 may store software (hereinafter referred to as‘MAC software’) including at least some functions of the MAC layer. TheMAC software processing unit 12 may execute the MAC software toimplement some functions of the MAC layer, and the MAC hardwareprocessing unit 13 may implement the remaining functions of the MAClayer as hardware (hereinafter referred to as ‘MAC hardware’). However,embodiments of the MAC processor 11 are not limited to this distributionof functionality.

The PHY processor 15 includes a transmitting (TX) signal processing unit100 and a receiving (RX) signal processing unit 200.

The baseband processor 10, the memory 40, the input interface unit 50,and the output interface unit 60 may communicate with one another viathe bus 70.

The RF transceiver 20 includes an RF transmitter 21 and an RF receiver22.

The memory 40 may further store an Operating System (OS) andapplications. The input interface unit 50 receives information from auser, and the output interface unit 60 outputs information to the user.

The antenna unit 30 includes one or more antennas. When Multiple inputMultiple Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antennaunit 30 may include a plurality of antennas.

FIG. 2 is a schematic block diagram of an exemplary transmission signalprocessor in a WLAN.

Referring to FIG. 2, the transmitting signal processing unit 100 mayinclude an encoder 110, an interleaver 120, a mapper 130, an InverseFourier Transformer (IFT) 140, and a Guard Interval (GI) inserter 150.

The encoder 110 encodes input data. For example, the encoder 100 may bea Forward Error Correction (FEC) encoder. The FEC encoder may include aBinary Convolutional Code (BCC) encoder followed by a puncturing device,or the FEC encoder may include a Low-Density Parity-Check (LDPC)encoder.

The transmitting signal processing unit 100 may further include ascrambler for scrambling the input data before encoding to reduce theprobability of long sequences of 0s or 1s. If BCC encoding is used inthe encoder 110, the transmitting signal processing unit 100 may furtherinclude an encoder parser for demultiplexing the scrambled bits among aplurality of BCC encoders. If LDPC encoding is used in the encoder 110,the transmitting signal processing unit 100 may not use the encoderparser.

The interleaver 120 interleaves the bits of each stream output from theencoder 110 to change the order of bits. Interleaving may be appliedonly when BCC encoding is used in the encoder 110. The mapper 130 mapsthe sequence of bits output from the interleaver 120 to constellationpoints. If LDPC encoding is used in the encoder 110, the mapper 130 mayfurther perform LDPC tone mapping in addition to constellation mapping.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may use a plurality of interleavers 120 and a plurality of mappers130 corresponding to the number of spatial streams, N_(SS). In thiscase, the transmitting signal processing unit 100 may further include astream parser for dividing outputs of the BCC encoders or output of theLDPC encoder into blocks that are sent to different interleavers 120 ormappers 130. The transmitting signal processing unit 100 may furtherinclude a Space-Time Block Code (STBC) encoder for spreading theconstellation points from the N_(SS) spatial streams into N_(STS)space-time streams and a spatial mapper for mapping the space-timestreams to transmit chains. The spatial mapper may use direct mapping,spatial expansion, or beamforming.

The IFT 140 converts a block of constellation points output from themapper 130 or the spatial mapper to a time-domain block (i.e., a symbol)by using Inverse Discrete Fourier Transform (IDFT) or Inverse FastFourier Transform (IFFT). If the STBC encoder and the spatial mapper areused, the IFT 140 may be provided for each transmit chain.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may insert Cyclic Shift Diversities (CSDs) to prevent unintentionalbeamforming. The CSD insertion may occur before or after IFT. The CSDmay be specified per transmit chain or may be specified per space-timestream. Alternatively, the CSD may be applied as a part of the spatialmapper.

When MU-MIMO is used, some blocks before the spatial mapper may beprovided for each user.

The GI inserter 150 prepends a GI to the symbol. The transmitting signalprocessing unit 100 may optionally perform windowing to smooth edges ofeach symbol after inserting the GI. The RF transmitter 21 converts thesymbols into an RF signal and transmits the RF signal via the antennaunit 30. When MIMO or MU-MIMO is used, the GI inserter 150 and the RFtransmitter 21 may be provided for each transmit chain.

FIG. 3 is a schematic block diagram of an exemplary a receiving signalprocessor in a WLAN.

Referring to FIG. 3, the receiving signal processing unit 200 includes aGI remover 220, a Fourier Transformer (FT) 230, a demapper 240, adeinterleaver 250, and a decoder 260.

An RF receiver 22 receives an RF signal via the antenna unit 30 andconverts the RF signal into one or more symbols. The GI remover 220removes the GI from the symbol. When MIMO or MU-MIMO is used, the RFreceiver 22 and the GI remover 220 may be provided for each receivechain.

The FT 230 converts the symbol (i.e., the time-domain block) into ablock of constellation points by using a Discrete Fourier Transform(DFT) or a Fast Fourier Transform (FFT). The FT 230 may be provided foreach receive chain.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may use/include a spatial demapper for converting Fourier Transformedreceiver chains to constellation points of the space-time streams, andan STBC decoder for despreading the constellation points from thespace-time streams into the spatial streams.

The demapper 240 demaps the constellation points output from the FT 230or the STBC decoder to bit streams. If LDPC encoding is applied to thereceived signal, the demapper 240 may further perform LDPC tonedemapping before constellation demapping. The deinterleaver 250deinterleaves the bits of each stream output from the demapper 240.Deinterleaving may be applied only when a BCC encoding scheme is appliedto the received signal.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may use a plurality of demappers 240 and a plurality of deinterleavers250 corresponding to the number of spatial streams. In this case, thereceiving signal processing unit 200 may further include a streamdeparser for combining streams output from the deinterleavers 250.

The decoder 260 decodes the streams output from the deinterleaver 250 orthe stream deparser. For example, the decoder 100 may be an FEC decoder.The FEC decoder may include a BCC decoder or an LDPC decoder. Thereceiving signal processing unit 200 may further include a descramblerfor descrambling the decoded data. If BCC decoding is used in thedecoder 260, the receiving signal processing unit 200 may furtherinclude an encoder deparser for multiplexing the data decoded by aplurality of BCC decoders. If LDPC decoding is used in the decoder 260,the receiving signal processing unit 200 may not use the encoderdeparser.

In a WLAN system, Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) is a basic MAC access mechanism. The CSMA/CA mechanism isreferred to as Distributed Coordination Function (DCF) of IEEE 802.11MAC, shortly as a ‘listen before talk’ access mechanism. According tothe CSMA/CA mechanism, an AP and/or a STA may sense a medium or achannel for a predetermined time before starting transmission, that is,may perform Clear Channel Assessment (CCA). If the AP or the STAdetermines that the medium or channel is idle, it may start to transmita frame on the medium or channel. On the other hand, if the AP and/orthe STA determines that the medium or channel is occupied or busy, itmay set a delay period (e.g., a random backoff period), wait for thedelay period without starting transmission, and then attempt to transmita frame. By applying a random backoff period, a plurality of STAs areexpected to attempt frame transmission after waiting for different timeperiods, resulting in minimizing collisions.

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs).

WLAN devices may exchange data frames, control frames, and managementframes with each other.

A data frame is used for transmission of data forwarded to a higherlayer. The WLAN device transmits the data frame after performing backoffif a Distributed Coordination Function IFS (DIFS) has elapsed from atime when the medium has been idle. A management frame is used forexchanging management information which is not forwarded to the higherlayer. The WLAN device transmits the management frame after performingbackoff if an IFS such as the DIFS or a Point Coordination Function IFS(PIFS) has elapsed. Subtype frames of the management frame include abeacon frame, an association request/response frame, a proberequest/response frame, and an authentication request/response frame. Acontrol frame is used for controlling access to the medium. Subtypeframes of the control frame include a Request-To-Send (RTS) frame, aClear-To-Send (CTS) frame, and an ACKnowledgement (ACK) frame. In thecase that the control frame is not a response frame to a previous frame,the WLAN device transmits the control frame after performing backoff ifthe DIFS has elapsed. In case that the control frame is a response frameto a previous frame, the WLAN device transmits the control frame withoutperforming backoff if a Short IFS (SIFS) has elapsed. The type andsubtype of a frame may be identified by a type field and a subtype fieldin a Frame Control (FC) field.

On the other hand, a Quality of Service (QoS) STA transmits a frameafter performing backoff if an Arbitration IFS (AIFS) for an associatedAccess Category (AC), i.e., AIFS[i] (i is determined based on AC) haselapsed. In this case, the AIFC[i] may be used for a data frame, amanagement frame, or a control frame that is not a response frame.

In the example illustrated in FIG. 4, upon generation of a frame to betransmitted, a STA may transmit the frame immediately, if it determinesthat the medium is idle for the DIFS or AIFS[i] or longer. The medium isbusy for a time period during which the STA transmits the frame. Duringthe time period, upon generation of a frame to be transmitted, anotherSTA may defer access by confirming that the medium is busy. If themedium gets idle, the STA that intends to transmit the frame may performa backoff operation after a predetermined IFS in order to minimizecollision with any other STA. Specifically, the STA that intends totransmit the frame selects a random backoff count, waits for a slot timecorresponding to the selected random backoff count, and then attempttransmission. The random backoff count is determined based on aContention Window (CW) parameter and the medium is monitoredcontinuously during count-down of backoff slots (i.e. decrement abackoff count-down) according to the determined backoff count. If theSTA monitors the medium as busy, the STA discontinues the count-down andwaits, and then, if the medium gets idle, the STA resumes thecount-down. If the backoff slot count reaches 0, the STA may transmitthe next frame.

FIG. 5 is a conceptual diagram illustrating a CSMA/CA-based frametransmission procedure for avoiding collisions between frames in achannel.

Referring FIG. 5, a first STA (STA1) is a transmit WLAN device fortransmitting data, a second STA (STA2) is a receive WLAN device forreceiving the data from STA1, and a third STA (STA3) is a WLAN devicewhich may be located in an area where a frame transmitted from STA1and/or a frame transmitted from STA2 can be received by STA3.

STA1 may determine whether the channel is busy by carrier sensing. TheSTA1 may determine the channel occupation based on an energy level onthe channel or correlation of signals in the channel, or may determinethe channel occupation by using a Network Allocation Vector (NAV) timer.

After determining that the channel is not being used by other devicesduring DIFS (that is, the channel is idle), STA1 may transmit an RTSframe to STA2 after performing backoff. Upon receiving the RTS frame,STA2 may transmit a CTS frame as a response to the CTS frame after SIFS.

When STA3 receives the RTS frame, STA3 may set the NAV timer for atransmission duration of subsequently transmitted frame by usingduration information included in the RTS frame. For example, the NAVtimer may be set for a duration of SIFS+CTS frame duration+SIFS+dataframe duration+SIFS+ACK frame duration. When STA3 receives the CTSframe, it may set the NAV timer for a transmission duration ofsubsequently transmitted frames by using duration information includedin the CTS frame. For example, the NAV timer may be set for a durationof SIFS+a data frame duration+SIFS+an ACK frame duration. Upon receivinga new frame before the NAV timer expires, STA3 may update the NAV timerby using duration information included in the new frame. STA3 does notattempt to access the channel until the NAV timer expires.

When STA1 receives the CTS frame from STA2, it may transmit a data frameto STA2 after SIFS elapsed from the CTS frame has been completelyreceived. Upon successfully receiving the data frame, STA2 may transmitan ACK frame as a response to the data frame after SIFS elapsed.

When the NAV timer expires, STA3 may determine whether the channel isbusy through the use of carrier sensing. Upon determining that thechannel is not in use by other devices during DIFS and after the NAVtimer has expired, STA3 may attempt channel access after a contentionwindow after a random backoff has elapsed.

FIG. 6 depicts an exemplary frame structure in a WLAN system.

PHY layer may prepare a transmission MAC PDU (MPDU) in response to aninstruction (or a primitive, which is a set of instructions or a set ofparameters) by the MAC layer. For example, upon receipt of aninstruction requesting transmission start from the MAC layer, the PHYlayer may switch to a transmission mode, construct a frame withinformation (e.g., data) received from the MAC layer, and transmit theframe.

Upon detection of a valid preamble in a received frame, the PHY layermonitors a header of the preamble and transmits an instructionindicating reception start of the PHY layer to the MAC layer.

Information is transmitted and received in frames in the WLAN system.For this purpose, a Physical layer Protocol Data Unit (PPDU) frameformat is defined.

A PPDU frame may include a Short Training Field (STF) field, a LongTraining Field (LTF) field, a SIGNAL (SIG) field, and a Data field. Themost basic (e.g., a non-High Throughput (non-HT)) PPDU frame may includeonly a Legacy-STF (L-STF) field, a Legacy-LTF (L-LTF) field, a SIGfield, and a Data field. Additional (or other types of) STF, LTF, andSIG fields may be included between the SIG field and the Data fieldaccording to the type of a PPDU frame format (e.g., an HT-mixed formatPPDU, an HT-greenfield format PPDU, a Very High Throughput (VHT) PPDU,etc.).

The STF is used for signal detection, Automatic Gain Control (AGC),diversity selection, fine time synchronization, etc. The LTF field isused for channel estimation, frequency error estimation, etc. The STFand the LTF fields may be referred to as signals for OFDM PHY layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about a modulation scheme and coding rateof data. The LENGTH field may include information about the length ofthe data. The SIG field may further include parity bits, SIG TAIL bits,etc.

The Data field may include a SERVICE field, a Physical layer ServiceData Unit (PSDU), and PPDU TAIL bits. When needed, the Data field mayfurther include padding bits. A part of the bits of the SERVICE fieldmay be used for synchronization at a descrambler of a receiver. The PSDUcorresponds to a MAC PDU defined at the MAC layer and may include datagenerated/used in a higher layer. The PPDU TAIL bits may be used toreturn an encoder to a zero state. The padding bits may be used to matchthe length of the Data filed in predetermined units.

A MAC PDU is defined according to various MAC frame formats. A basic MACframe includes a MAC header, a frame body, and a Frame Check Sequence(FCS). The MAC frame includes a MAC PDU and may be transmitted andreceived in the PSDU of the data part in the PPDU frame format.

The MAC header includes a Frame Control field, a Duration/Identifier(ID) field, an Address field, etc. The Frame Control field may includecontrol information required for frame transmission/reception. TheDuration/ID field may be set to a time for transmitting the frame. Fordetails of Sequence Control, QoS Control, and HT Control subfields ofthe MAC header, refer to the IEEE 802.11-2012 technical specification.

The Frame Control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order subfields. For the contents ofeach subfield in the Frame Control field, refer to the IEEE 802.11-2012technical specification.

A Null-Data Packet (NDP) frame format is a frame format that does notinclude a data packet. In other words, the NDP frame format includesonly a Physical Layer Convergence Protocol (PLCP) header part (i.e., theSTF, LTF, and SIG fields) of the general PPDU frame format, without theremaining part (i.e., the Data field) of the general PPDU frame format.The NDP frame format may be referred to as a short frame format.

The IEEE 802.11ax task group is discussing a WLAN system, called a HighEfficiency WLAN (HEW) system, that operates in 2.4 GHz or 5 GHz andsupports a channel bandwidth (or channel width) of 20 MHz, 40 MHz, 80MHz, or 160 MHz. The present disclosure defines a new PPDU frame formatfor the IEEE 802.11ax HEW system. The new PPDU frame format may supportMU-MIMO or OFDMA. A PPDU of the new format may be referred to as a ‘HEWPPDU’ or ‘HE PPDU’ (similarly, HEW xyz may be referred to as ‘HE xyz’ or‘HE-xyz’ in the following descriptions).

In present specification, the term ‘MU-MIMO or OFDMA mode’ includesMU-MIMO without using OFDMA, or OFDMA mode without using MU-MIMO in anorthogonal frequency resource, or OFDMA mode using MU-MIMO in anorthogonal frequency resource.

FIG. 7 depicts an exemplary HE PPDU frame format.

A transmitting STA may generate a PPDU frame according to the HE PPDUframe format as illustrated in FIG. 7 and transmit the PPDU frame to areceiving STA. The receiving STA may receive, detect, and process thePPDU.

The HE PPDU frame format may broadly include two parts: the first partincluding an L-STF field, an L-LTF field, an L-SIG field, an RL-SIGfield, a HE-SIG-A field, and a HE-SIG-B field and the second partincluding a HE-STF field, a HE-LTF field, and a HE-DATA field. 64-FFTbased on a channel bandwidth of 20 MHz may be applied to the first partand a basic subcarrier spacing of 312.5 kHz and a basic DFT period of3.2 μs may be included in the first part. 256-FFT based on a channelbandwidth of 20 MHz may be applied to the second part and a basicsubcarrier spacing of 75.125 kHz and a basic DFT period of 12.8 μs maybe included in the second part.

The HE-SIG-A field may include N_(HESIGA) symbols, the HE-SIG-B fieldmay include N_(HESIGB) symbols, the HE-LTF field may include N_(HELTF)symbols, and the HE-DATA field may include N_(DATA) symbols.

A detailed description of the fields included in the HE PPDU frameformat is given in Table 1.

TABLE 1 DFT Subcarrier Element definition duration period GI spacingDescription Legacy(L)- Non-high 8 μs — — equivalent to L-STF of anon-trigger-based STF throughput(HT) 1,250 kHz PPDU has a periodicity of0.8 μs Short Training with 10 periods. field L-LTF Non-HT Long 8 μs 3.2μs 1.6 μs 312.5 kHz Training field L-SIG Non-HT 4 μs 3.2 μs 0.8 μs 312.5kHz SIGNAL field RL-SIG Repeated Non- 4 μs 3.2 μs 0.8 μs 312.5 kHz HTSIGNAL field HE-SIG-A HE SIGNAL A N_(HESIGA) * 3.2 μs 0.8 μs 312.5 kHzHE-SIG-A is duplicated on each field 4 μs 20 MHz segment after thelegacy preamble to indicate common control information. N_(HESIGA) meansthe number of OFDM symbols of the HE-SIG-A field and is equal to 2 or 4.HE-SIG-B HE SIGNAL B N_(HESIGB) * 3.2 μs 0.8 μs 312.5 kHz N_(HESIGB)means the number of field 4 μs OFDM symbols of the HE-SIG-B field and isvariable. DL MU packet contains HE-SIG- B. SU packets and UL Triggerbased packets do not contain HE-SIG-B. HE-STF HE Short 4 or 8 μs — —non- HE-STF of a non-trigger-based Training field trigger- PPDU has aperiodicity of 0.8 μs based with 5 periods. A non-trigger-based PPDU:PPDU is not sent in response to a (equivalent trigger frame. to) 1,250kHz; The HE-STF of a trigger-based trigger- PPDU has a periodicity of1.6 μs based with 5 periods. A trigger-based PPDU: PPDU is an UL PPDUsent in (equivalent response to a trigger frame. to) 625 kHz HE-LTF HELong N_(HELTF) * 2xLTF: supports 2xLTF: HE PPDU shall support 2xLTFTraining field (DTF 6.4 μs 0.8, 1.6, (equivalent mode and 4xLTF mode.period + 4xLTF: 3.2 μs to) 156.25 kHz; In the 2xLTF mode, HE-LTF GI) μs12.8 μs 4xLTF: symbol excluding GI is equivalent 78.125 kHz tomodulating every other tone in an OFDM symbol of 12.8 μs excluding GI,and then removing the second half of the OFDM symbol in time domain.N_(HELTF) means the number of HE- LTF symbols and is equal to 1, 2, 4,6, 8. HE-DATA HE DATA N_(DATA) * 12.8 μs supports 78.125 kHz N_(DATA)means the number of HE field (DTF 0.8, 1.6, data symbols. period + 3.2μs GI) μs

L-STF is a non-HT Short Training field and may have a duration of 8 μsand a subcarrier spacing equivalent to 1250 kHz. L-STF of a PPDU whichis not based on a trigger may have a periodicity of 0.8 μs with 10periods. Herein, the trigger corresponds to scheduling information forUL transmission.

L-LTF is a non-HT Long Training field and may have a duration of 8 μS, aDFT period of 3.2 μs, a Guard Interval (GI) of 1.6 μs, and a subcarrierspacing of 312.5 kHz.

L-SIG is a non-HT SIGNAL field and may have a duration of 4 μs, a DFTperiod of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of 312.5 kHz.

RL-SIG is a Repeated Non-HT SIGNAL field and may have a duration of 4its, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz.

L-STF, L-LTF, L-SIG, and RL-SIG may be called legacy preambles.

HE-SIG-A is a HE SIGNAL A field and may have a duration of N_(HESIGA)*4μs, a DFT period of 3.2 μS, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. HE-SIG-A may be duplicated on each 20 MHz segment after thelegacy preambles to indicate common control information. N_(HESIGA)represents the number of OFDM symbols of the HE-SIG-A field and may havea value of 2 or 4.

HE-SIG-B is a HE SIGNAL B field and may have a duration of N_(HESIGB)*4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. N_(HESIGB) represents the number of OFDM symbols of theHE-SIG-B field and may have a variable value. In addition, although a DLMulti-User (MU) packet may include the HE-SIG-B field, a Single-User(SU) packet and a UL trigger based packet may not include the HE-SIG-Bfield.

HE-STF is a HE Short Training field and may have a duration of 4 or 8μs. A non-trigger based PPDU may have a subcarrier spacing equivalent to1250 kHz and a trigger based PPDU may have a subcarrier spacingequivalent to 625 kHz. HE-STF of the non-triggered PPDU may have aperiodicity of 0.8 μs with 4 periods. The non-triggered PPDU is nottransmitted in response to a trigger field. HE-STF of the trigger basedPPDU may have a periodicity of 1.6 μs with 5 periods. The trigger basedPPDU is a UL PPDU transmitted in response to the trigger frame.

HE-LTF is a HE Long Training field and may have a duration ofN_(HELTF)*(DFT period+GI)μs. N_(HELTF) represents the number of HE-LTFsymbols and may have a value of 1, 2, 4, 6, or 8. A HE PPDU may supporta 2×LTF mode and a 4×LTF mode. In the 2×LTF mode, a HE-LTF symbol exceptfor a GI is equivalent to a symbol obtained by modulating every othertone in an OFDM symbol of 12.8 μs excluding a GI and then eliminatingthe first half or the second half of the OFDM symbol in the time domain.In the 4×LTF mode, a HE-LTF symbol excluding a GI are equivalent to asymbol obtained by modulating every fourth tone in an OFDM symbol of12.8 μs excluding a GI and then eliminating the first three-fourths orthe last three-fourths of the OFDM symbol in the time domain. 2×LTF mayhave a DFT period of 6.4 μs and 4×LTF may have a DFT period of 12.8 μs.A GI of HE-LTF may support 0.8 μs, 1.6 μs, and 3.2 μs. 2×LTF may have asubcarrier spacing equivalent to 156.25 kHz and 4×LTF may have asubcarrier spacing of 78.125 kHz.

HE-DATA is a HE DATA field and may have a duration of, N_(DATA)*(DFTperiod+GI)μs. N_(DATA) represents the number of HE-DATA symbols. HE-DATAmay have a DFT period of 12.8 μs. A GI of HE-DATA may support 0.8 μs,1.6 μs, and 3.2 μs. HE-DATA may have a subcarrier spacing of 78.125 kHz.

The above description of the fields included in the HE PPDU frame formatmay be combined with exemplary HE PPDU frame formats described below.For example, characteristics of fields exemplarily described below maybe applied while a transmission order of the fields of the HE PPDU frameformat of FIG. 7 is maintained.

FIG. 8 depicts an exemplary HE PPDU frame format according to thepresent disclosure.

Referring to FIG. 8, the vertical axis represents frequency and thehorizontal axis represents time. It is assumed that frequency and timeincrease in the upward direction and the right direction, respectively.

In the example of FIG. 8, one channel includes four subchannels. AnL-STF, an L-LTF, an L-SIG, and an HE-SIG-A may be transmitted perchannel (e.g., 20 MHz), a HE-STF and a HE-LTF may be transmitted on eachsubchannel being a basic subchannel unit (e.g., 5 MHz), and a HE-SIG-Band a PSDU may be transmitted on each of subchannels allocated to a STA.A subchannel allocated to a STA may have a size required for PSDUtransmission to the STA. The size of the subchannel allocated to the STAmay be N (N=1, 2, 3, . . . ) times as large as the size of basicsubchannel unit (i.e., a subchannel having a minimum size). In theexample of FIG. 8, the size of a subchannel allocated to each STA isequal to the size of the basic subchannel unit. For example, a firstsubchannel may be allocated for PSDU transmission from an AP to STA1 andSTA2, a second subchannel may be allocated for PSDU transmission fromthe AP to STA3 and STA4, a third subchannel may be allocated for PSDUtransmission from the AP to STA5, and a fourth subchannel may beallocated for PSDU transmission from the AP to STA6.

While the term subchannel is used in the present disclosure, the termsubchannel may be referred to as Resource Unit (RU) or subband. Inparticular, the terms like OFDMA subchannel, OFDMA RU, OFDMA subband canbe used in embodiments for OFDMA in the present disclosure. Terms like abandwidth of a subchannel, a number of tones (or subcarriers) allocatedto a subchannel, a number of data tones (or data subcarriers) allocatedto a subchannel can be used to express a size of a subchannel. Asubchannel refers to a frequency band allocated to a STA and a basicsubchannel unit refers to a basic unit used to represent the size of asubchannel. While the size of the basic subchannel unit is 5 MHz in theabove example, this is purely exemplary. Thus, the basic subchannel unitmay have a size of 2.5 MHz.

In FIG. 8, a plurality of HE-LTF elements are distinguished in the timeand frequency domains. One HE-LTF element may correspond to one OFDMsymbol in time domain and one subchannel unit (i.e., a subchannelbandwidth allocated to a STA) in frequency domain. The HE-LTF elementsare logical units, and the PHY layer does not necessarily operate inunits of an HE-LTF element. In the following description, a HE-LTFelement may be referred to shortly as a HE-LTF.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in time domain and in one channel unit (e.g., 20 MHz) infrequency domain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in time domain and in one subchannel unit (i.e., asubchannel bandwidth allocated to a STA) in frequency domain.

A HE-LTF field may be a set of HE-LTF elements, HE-LTF symbols, orHE-LTF sections for a plurality of stations.

The L-STF field is used for frequency offset estimation and phase offsetestimation, for preamble decoding at a legacy STA (i.e., a STA operatingin a system such as IEEE 802.11a/b/g/n/ac). The L-LTF field is used forchannel estimation, for the preamble decoding at the legacy STA. TheL-SIG field is used for the preamble decoding at the legacy STA andprovides a protection function for PPDU transmission of a third-partySTA (e.g., a third-party STA is not allowed to transmit during a certainperiod based on the value of a LENGTH field included in the L-SIGfield).

HE-SIG-A (or HEW SIG-A) represents High Efficiency Signal A (or HighEfficiency WLAN Signal A), and includes HE PPDU (or HEW PPDU) modulationparameters, etc. for HE preamble (or HEW preamble) decoding at a HE STA(or HEW STA). The parameters set included in the HEW SIG-A field mayinclude one or more of Very High Throughput (VHT) PPDU modulationparameters transmitted by IEEE 802.11ac stations, as listed in [Table 2]below, to ensure backward compatibility with legacy STAs (e.g., IEEE802.11 ac stations).

TABLE 2 Two parts of Number VHT-SIG-A Bit Field of bits DescriptionVHT-SIG-A1 B0-B1 BW 2 Set to 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz,and 3 for 160 MHz and 80 + 80 MHz B2 Reserved 1 Reserved. Set to 1. B3STBC 1 For a VHT SU PPDU:  Set to 1 if space time block coding is usedand set to 0  otherwise. For a VHT MU PPDU:  Set to 0. B4-B9 Group ID 6Set to the value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a VHT SU PPDU; otherwise, indicates a VHT MU PPDU. B10-B21NSTS/Partial 12 For a VHT MU PPDU: NSTS is divided into 4 user AIDpositions of 3 bits each. User position p, where 0 ≦ p ≦ 3, uses bitsB(10 + 3p) to B(12 + 3p). The number of space- time streams for user uare indicated at user position p = USER_POSITION[u] where u = 0, 1, . .. , NUM_USERS − 1 and the notation A[b] denotes the value of array A atindex b. Zero space-time streams are indicated at positions not listedin the USER_POSITION array. Each user position is set as follows:  Setto 0 for 0 space-time streams  Set to 1 for 1 space-time stream  Set to2 for 2 space-time streams  Set to 3 for 3 space-time streams  Set to 4for 4 space-time streams  Values 5-7 are reserved For a VHT SU PPDU:B10-B12  Set to 0 for 1 space-time stream  Set to 1 for 2 space-timestreams  Set to 2 for 3 space-time streams  Set to 3 for 4 space-timestreams  Set to 4 for 5 space-time streams  Set to 5 for 6 space-timestreams  Set to 6 for 7 space-time streams  Set to 7 for 8 space-timestreams B13-B21  Partial AID: Set to the value of the TXVECTOR parameter PARTIAL_AID. Partial AID provides an  abbreviated indicationof the intended recipient(s) of the  PSDU (see 9.17a). B22TXOP_PS_NOT_ALLOWED 1 Set to 0 by VHT AP if it allows non-AP VHT STAs inTXOP power save mode to enter Doze state during a TXOP. Set to 1otherwise. The bit is reserved and set to 1 in VHT PPDUs transmitted bya non-AP VHT STA. B23 Reserved 1 Set to 1 VHT-SIG-A2 B0 Short G1 1 Setto 0 if short guard interval is not used in the Data field. Set to 1 ifshort guard interval is used in the Data field. B1 Short G1 1 Set to 1if short guard interval is used and N_(SYM) mod 10 = 9; N_(SYM)otherwise, set to 0. N_(SYM) is defined in 22.4.3. Disambiguation B2SU/MU[0] 1 For a VHT SU PPDU, B2 is set to 0 for BCC, 1 for LDPC CodingFor a VHT MU PPDU, if the MU[0] NSTS field is nonzero, then B2 indicatesthe coding used for user u with USER_POSITION[u] = 0; set to 0 for BCCand 1 for LDPC. If the MU[0] NSTS field is 0, then this field isreserved and set to 1. B3 LDPC Extra 1 Set to 1 if the LDPC PPDUencoding process (if an SU OFDM PPDU), or at least one LDPC user's PPDUencoding process Symbol (if a VHT MU PPDU), results in an extra OFDMsymbol (or symbols) as described in 22.3.10.5.4 and 22.3.10.5.5. Set to0 otherwise. B4-B7 SU VHT- 4 For a VHT SU PPDU: MCS/MU[1-3]  VHT-MCSindex Coding For a VHT MU PPDU:  If the MU[1] NSTS field is nonzero,then B4 indicates  coding for user u with USER_POSITION[u] = 1: set to 0 for BCC, 1 for LDPC. If the MU[1] NSTS field is 0, then  B4 is reservedand set to 1.  If the MU[2] NSTS field is nonzero, then B5 indicates coding for user u with USER_POSITION[u] = 2: set to 0  for BCC, 1 forLDPC. If the MU[2] NSTS field is 0, then  B5 is reserved and set to 1. If the MU[3] NSTS field is nonzero, then B6 indicates  coding for useru with USER_POSITION[u] = 3: set to 0  for BCC, 1 for LDPC. If the MU[3]NSTS field is 0, then  B6 is reserved and set to 1.  B7 is reserved andset to 1 B8 Beamformed 1 For a VHT SU PPDU:  Set to 1 if a Beamformingsteering matrix is applied to the  waveform in an SU transmission asdescribed in  20.3.11.11.2, set to 0 otherwise. For a VHT MU PPDU: Reserved and set to 1 NOTE-If equal to 1 smoothing is not recommended.B9 Reserved 1 Reserved and set to 1 B10-B17 CRC 8 CRC calculated as in20.3.9.4.4 with c7 in B10. Bits 0-23 of HT-SIG1 and bits 0-9 of HT-SIG2are replaced by bits 0.23 of VHT-SIG-A1 and bits 0-9 of VHT-SIG-A2,respectively. B18-B23 Tail 6 Used to terminate the trellis of theconvolutional decoder. Set to 0.

[Table 2] illustrates fields, bit positions, numbers of bits, anddescriptions included in each of two parts, VHT-SIG-A1 and VHT-SIG-A2,of the VHT-SIG-A field defined by the IEEE 802.11ac standard. Forexample, a BW (BandWidth) field occupies two Least Significant Bits(LSBs), B0 and B1 of the VHT-SIG-A1 field and has a size of 2 bits. Ifthe 2 bits are set to 0, 1, 2, or 3, the BW field indicates 20 MHz, 40MHz, 80 MHz, or 160 and 80+80 MHz. For details of the fields included inthe VHT-SIG-A field, refer to the IEEE 802.11ac-2013 technicalspecification. In the HE PPDU frame format of the present disclosure,the HE-SIG-A field may include one or more of the fields included in theVHT-SIG-A field, and it may provide backward compatibility with IEEE802.11ac stations.

FIG. 9 depicts subchannel allocation in the HE PPDU frame formataccording to the present disclosure.

In FIG. 9, it is assumed that information indicating subchannelsallocated to STAs in HE PPDU indicates that 0 MHz subchannel isallocated to STA1 (i.e., no subchannel is allocated), a 5-MHz subchannelis allocated to each of STA2 and STA3, and a 10-MHz subchannel isallocated to STA4.

In the example of FIG. 9, an L-STF, an L-LTF, an L-SIG, and a HE-SIG-Amay be transmitted per channel (e.g., 20 MHz), a HE-STF and a HE-LTF maybe transmitted on each of subchannels being basic subchannel units(e.g., 5 MHz), and a HE-SIG-B and a PSDU may be transmitted on each ofsubchannels allocated to STAs. A subchannel allocated to a STA has asize required for PSDU transmission to the STA. The size of thesubchannel allocated to the STA may be an N (N=1, 2, 3, . . . ) multipleof the size of the basic subchannel unit (i.e., a minimum-sizesubchannel unit). In the example of FIG. 9, the size of a subchannelallocated to STA2 is equal to that of the basic subchannel unit, thesize of a subchannel allocated to STA3 is equal to that of the basicsubchannel unit, and the size of a subchannel allocated to STA4 is twicelarger than that of the basic subchannel unit.

FIG. 9 illustrates a plurality of HE-LTF elements and a plurality ofHE-LTF subelements which are distinguished in the time and frequencydomains. One HE-LTF element may correspond to one OFDM symbol in thetime domain and one subchannel unit (i.e., the bandwidth of a subchannelallocated to a STA) in the frequency domain. One HE-LTF subelement maycorrespond to one OFDM symbol in the time domain and one basicsubchannel unit (e.g. 5 MHz) in the frequency domain. In the example ofFIG. 9, one HE-LTF element includes one HE-LTF subelement in the 5-MHzsubchannel allocated to STA2 or STA3. On the other hand, one HE-LTFelement includes two HE-LTF subelements in the third subchannel, i.e.,10-MHz subchannel, allocated to STA4. A HE-LTF element and a HE-LTFsubelement are logical units and the PHY layer does not always operatein units of a HE-LTF element or HE-LTF subelement.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in the time domain and one channel unit (e.g. 20 MHz) in thefrequency domain. That is, one HE-LTF symbol may be divided into HE-LTFelements by a subchannel width allocated to a STA and into HE-LTFsubelements by the width of the basic subchannel unit in the frequencydomain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one subchannel unit (i.e. thebandwidth of a subchannel allocated to a STA) in the frequency domain. AHE-LTF subsection may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one basic subchannel unit(e.g., 5 MHz) in the frequency domain. In the example of FIG. 9, oneHE-LTF section includes one HE-LTF subsection in the 5-MHz subchannelallocated to STA2 or STA3. On the other hand, one HE-LTF sectionincludes two HE-LTF subsections in the third subchannel, i.e., 10-MHzsubchannel, allocated to STA4.

A HE-LTF field may correspond to a set of HE-LTF elements (orsubelements), HE-LTF symbols, or HE-LTF sections (or subsections) for aplurality of stations.

For the afore-described HE PPDU transmission, subchannels allocated to aplurality of HE STAs may be contiguous in the frequency domain. In otherwords, for HE PPDU transmission, the subchannels allocated to the HESTAs may be sequential and any intermediate one of the subchannels ofone channel (e.g., 20 MHz) may not be allowed to be unallocated orempty. Referring to FIG. 8, if one channel includes four subchannels, itmay not be allowed to keep the third subchannel unallocated and empty,while the first, second, and fourth subchannels are allocated to STAs.However, the present disclosure does not exclude non-allocation of anintermediate subchannel of one channel to a STA.

FIG. 10 depicts a subchannel allocation method according to the presentdisclosure.

In the example of FIG. 10, a plurality of contiguous channels (e.g.,20-MHz-bandwidth channels) and boundaries of the plurality of contiguouschannels are shown. In FIG. 10, a preamble may correspond to an L-STF,an L-LTF, an L-SIG, and a HE-SIG-A as illustrated in the examples ofFIGS. 8 and 9.

A subchannel for each HE STA may be allocated only within one channel,and may not be allocated with partially overlapping between a pluralityof channels. That is, if there are two contiguous 20-MHz channels CH1and CH2, subchannels for STAs paired for MU-MIMO-mode or OFDMA-modetransmission may be allocated either within CH1 or within CH2, and itmay be prohibited that one part of a subchannel exists in CH1 andanother part of the subchannel exists in CH2. This means that onesubchannel may not be allocated with crossing a channel boundary. Fromthe perspective of RUs supporting the MU-MIMO or OFDMA mode, a bandwidthof 20 MHz may be divided into one or more RUs, and a bandwidth of 40 MHzmay be divided into one or more RUs in each of two contiguous 20-MHzbandwidths, and no RU is allocated with crossing the boundary betweentwo contiguous 20-MHz bandwidths.

As described above, it is not allowed that one subchannel belongs to twoor more 20-MHz channels. Particularly, a 2.4-GHz OFDMA mode may supporta 20-MHz OFDMA mode and a 40-MHz OFDMA mode. In the 2.4-GHz OFDMA mode,it may not be allowed that one subchannel belongs to two or more 20-MHzchannels.

FIG. 10 is based on the assumption that subchannels each having the sizeof a basic subchannel unit (e.g., 5 MHz) in CH1 and CH2 are allocated toSTA1 to STA7, and subchannels each having double the size (e.g., 10 MHz)of the basic subchannel unit in CH4 and CH5 are allocated to STA8, STA9,and STA10.

As illustrated in the lower part of FIG. 9, although a subchannelallocated to STA1, STA2, STA3, STA5, STA6, or STA7 is fully overlappedonly with one channel (i.e., without crossing the channel boundary, orbelonging only to one channel), a subchannel allocated to STA4 ispartially overlapped with the two channels (i.e., crossing the channelboundary, or belonging to the two channels). In the forgoing example ofthe present disclosure, the subchannel allocation to STA4 is notallowed.

As illustrated in the upper part of FIG. 9, although a subchannelallocated to STA8 or STA10 is fully overlapped only with one channel(i.e., crossing the channel boundary, or belonging only to one channel),a subchannel allocated to STA9 is partially overlapped with two channels(i.e., crossing the channel boundary, or belonging to the two channels).In the forgoing example of the present disclosure, the subchannelallocation to STA9 is not allowed.

On the other hand, it may be allowed to allocate a subchannel partiallyoverlapped between a plurality of channels (i.e., crossing the channelboundary, or belonging to two channels). For example, in SU-MIMO modetransmission, a plurality of contiguous channels may be allocated to aSTA and any of one or more subchannels allocated to the STA may crossthe boundary between two contiguous channels.

While the following description is given with an assumption that onesubchannel has a channel bandwidth of 5 MHz in one channel having achannel bandwidth of 20 MHz, this is provided to simplify thedescription of the principle of the present disclosure and thus shouldnot be construed as limiting the present disclosure. For example, thebandwidths of a channel and a subchannel may be defined or allocated asvalues other than the above examples. In addition, a plurality ofsubchannels in one channel may have the same or different channelwidths.

FIG. 11 depicts the starting and ending points of a HE-LTF field in theHE PPDU frame format according to the present disclosure.

To support the MU-MIMO mode and the OFDMA mode, the HE PPDU frame formataccording to the present disclosure may include, in the HE-SIG-A field,information about the number of spatial streams to be transmitted to aHE STA allocated to each subchannel.

If MU-MIMO-mode or OFDMA-mode transmission is performed to a pluralityof HE STAs on one subchannel, the number of spatial streams to betransmitted to each of the HE STAs may be provided in the HE-SIG-A orHE-SIG-B field, which will be described later in detail.

FIG. 11 is based on the assumption that a first 5-MHz subchannel isallocated to STA1 and STA2 and two spatial streams are transmitted toeach STA in a DL MU-MIMO or OFDMA mode (i.e., a total of four spatialstreams are transmitted on one subchannel). For this purpose, a HE-STF,a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF, and a HE-SIG-B follow theHE-SIG-A field on the subchannel. The HE-STF is used for frequencyoffset estimation and phase offset estimation for the 5-MHz subchannel.The HE-LTFs are used for channel estimation for the 5-MHz subchannel.Since the subchannel carries four spatial streams, as many HE-LTFs(i.e., HE-LTF symbols or HE-LTF elements in a HE-LTF section) as thenumber of the spatial streams, that is, four HE-LTFs are required tosupport MU-MIMO transmission.

According to an example of the present disclosure, relationship betweena total number of spatial streams transmitted on one subchannel and anumber of HE-LTFs is listed in [Table 3].

TABLE 3 Total number of spatial streams transmitted on one subchannelNumber of HE-LTFs 1 1 2 2 3 4 4 4 5 6 6 6 7 8 8 8

Referring to [Table 3], if one spatial stream is transmitted on onesubchannel, at least one HE-LTF needs to be transmitted on thesubchannel. If an even number of spatial streams are transmitted on onesubchannel, at least as many HE-LTFs as the number of the spatialstreams need to be transmitted. If an odd number of spatial streamsgreater than one are transmitted on one subchannel, at least as manyHE-LTFs as a number of adding 1 to the number of the spatial streamsneed to be transmitted.

Referring to FIG. 11 again, it is assumed that the second 5-MHzsubchannel is allocated to STA3 and STA4 and one spatial streams per STAis transmitted in the DL MU-MIMO or OFDMA mode (i.e., a total of twospatial streams are transmitted on one subchannel). In this case, twoHE-LTFs need to be transmitted on the second subchannel, however, in theexample of FIG. 11, a HE-STF, a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF,and a HE-SIG-B follow the HE-SIG-A field on the subchannel (i.e., fourHE-LTFs are transmitted). This is for setting the same starting time ofPSDU transmission for subchannels allocated to other STAs paired withSTA3 and STA4 for MU-MIMO transmission. If only two HE-LTFs aretransmitted on the second subchannel, PSDUs are transmitted at differenttime points on the first and second subchannels. PSDU transmission oneach subchannel at a different time point results in discrepancy betweenOFDM symbol timings of subchannels, thereby no orthogonality ismaintained. To overcome this problem, an additional constraint need tobe imposed for HE-LTF transmission.

Basically, transmission of as many HE-LTFs as required is sufficient inan SU-MIMO or non-OFDMA mode. However, timing synchronization (oralignment) with fields transmitted on subchannels for other paired STAsis required in the MU-MIMO or OFDMA mode. Accordingly, the numbers ofHE-LTFs may be determined for all other subchannels based on asubchannel having the maximum number of streams in MU-MIMO-mode orOFDMA-mode transmission.

Specifically, the numbers of HE-LTFs may be determined for allsubchannels according to the maximum of the numbers of HE-LTFs (HE-LTFsymbols or HE-LTF elements in a HE-LTF section) required according tothe total numbers of spatial streams transmitted on each subchannel, fora set of HE STAs allocated to each subchannel. A “set of HE STAsallocated to each subchannel” is one HE STA in the SU-MIMO mode, and aset of HE STAs paired across a plurality of subchannels in the MU-MIMOmode. The ‘number of spatial streams transmitted on each subchannel’ isthe number of spatial streams transmitted to one HE STA in the SU-MIMOmode, and the number of spatial streams transmitted to a plurality of HESTAs paired on the subchannel in the MU-MIMO mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point in a HE PPDU for all users (i.e.HE STAs) in MU-MIMO-mode or OFDMA-mode transmission. Or it may be saidthat the lengths of HE-LTF sections are equal on a plurality ofsubchannels for all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-modetransmission. Or it may be said that the number of HE-LTF elementsincluded in each HE-LTF section is equal on a plurality of subchannelsfor all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized among aplurality of subchannels for all HE STAs in MU-MIMO-mode or OFDMA-modetransmission.

As described above, the number of HE-LTF symbols (refer to FIG. 8) maybe 1, 2, 4, 6, or 8 in HE PPDU transmission in the MU-MIMO or OFDMAmode, determined according to the maximum of the numbers of spatialstreams on each of a plurality of subchannels. A different number ofspatial streams may be allocated to each of a plurality of subchannels,and the number of spatial streams allocated to one subchannel is thenumber of total spatial streams for all users allocated to thesubchannel. That is, the number of HE-LTF symbols may be determinedaccording to the number of spatial streams allocated to a subchannelhaving a maximum number of spatial streams by comparing the number oftotal spatial streams for all users allocated to one of a plurality ofsubchannels with the number of total spatial streams for all usersallocated to another subchannel.

Specifically, in HE PPDU transmission in the OFDMA mode, the number ofHE-LTF symbols may be 1, 2, 4, 6, or 8, determined based on the numberof spatial streams transmitted in a subchannel having a maximum numberof spatial streams across a plurality of subchannels. Further, in HEPPDU transmission in the OFDMA mode, the number of HE-LTF symbols may bedetermined based on whether the number of spatial streams transmitted ina subchannel having a maximum number of spatial streams across aplurality of subchannels is odd or even (refer to [Table 3]). That is,in HE PPDU transmission in the OFDMA mode, when the number (e.g., K) ofspatial streams transmitted in a subchannel having a maximum number ofspatial streams across a plurality of subchannels is an even number, thenumber of HE-LTF symbols may be equal to K. In HE PPDU transmission inthe OFDMA mode, when the number, K, of spatial streams transmitted in asubchannel having a maximum number of spatial streams across a pluralityof subchannels is an odd number greater than one, the number of HE-LTFsymbols may be equal to K+1.

When only one STA is allocated to one subchannel in OFDMA mode (i.e.,OFDMA mode without using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of spatial streams for a STA allocated to each subchannel.When more than one STA is allocated to one subchannel in OFDMA mode(i.e., OFDMA mode using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of STAs allocated to each subchannel and the number ofspatial streams for each STA allocated to each subchannel (e.g., if STA1and STA2 are allocated to one subchannel, sum of the number of spatialstreams for STA1 and the number of spatial streams for STA2).

When transmitting a HE PPDU frame in the MU-MIMO or OFDMA mode, atransmitter may generate P (P is an integer equal to or larger than 1)HE-LTF symbols (refer to FIG. 8) and transmit a HE PPDU frame includingat least the P HE-LTF symbols and a Data field to a receiver. The HEPPDU frame may be divided into Q subchannels in the frequency domain (Qis an integer equal to or larger than 2). Each of the P HE-LTF symbolsmay be divided into Q HE-LTF elements corresponding to the Q subchannelsin the frequency domain. That is, the HE PPDU may include P HE-LTFelements on one subchannel (herein, the P HE-LTF elements may belong toone HE-LTF section on the subchannel).

As described above, the number of HE-LTF elements (i.e., P) in one ofthe Q subchannels may be equal to the number of HE-LTF elements (i.e. P)of another subchannel. Also, the number of HE-LTF elements (i.e., P)included in a HE-LTF section in one of the Q subchannels may be equal tothe number of HE-LTF elements (i.e. P) included in a HE-LTF section inanother subchannel. The HE-LTF section of one of the Q subchannels maystart and end at the same time points as the HE-LTF section of anothersubchannel. Also, the HE-LTF sections may start and end at the same timepoints across the Q subchannels (i.e., across all users or stations).

Referring to FIG. 11 again, the third 5-MHz subchannel is allocated toSTA5 and one spatial stream is transmitted on the subchannel in SU-MIMO(considering all subchannels, a plurality of spatial streams aretransmitted to STA1 to STA6 in MU-MIMO or OFDMA mode). In this case,although transmission of one HE-LTF is sufficient for the subchannel, asmany HE-LTFs as the maximum of the numbers of HE-LTFs on the othersubchannels, that is, four HE-LTFs are transmitted on the subchannel inorder to align the starting points and ending points of the HE-LTFfields of the subchannels.

The fourth 5-MHz subchannel is allocated to STA6 and one spatial streamis transmitted on the subchannel in SU-MIMO (considering all othersubchannels, a plurality of spatial streams are transmitted to STA1 toSTA6 in MU-MIMO or OFDMA mode). In this case, although transmission ofone HE-LTF is sufficient for the subchannel, as many HE-LTFs as themaximum of the numbers of HE-LTFs on the other subchannels, that is,four HE-LTFs are transmitted on the subchannel in order to align thestarting points and ending points of the HE-LTF fields of thesubchannels.

In the example of FIG. 11, the remaining two HE-LTFs except two HE-LTFsrequired for channel estimation of STA3 and STA4 on the secondsubchannel, the remaining three HE-LTFs except one HE-LTF required forchannel estimation of STA5 on the third subchannel, and the remainingthree HE-LTFs except one HE-LTF required for channel estimation of STA6on the fourth subchannel may be said to be placeholders that areactually not used for channel estimation at the STAs.

FIG. 12 depicts a HE-SIG-B field and a HE-SIG-C field in the HE PPDUframe format according to the present disclosure.

To effectively support MU-MIMO-mode or OFDMA-mode transmission in the HEPPDU frame format according to the present disclosure, independentsignaling information may be transmitted on each subchannel.Specifically, a different number of spatial streams may be transmittedto each of a plurality of HE STAs that receive an MU-MIMO-mode orOFDMA-mode transmission simultaneously. Therefore, information about thenumber of spatial streams to be transmitted should be indicated to eachHE STA.

Information about the number of spatial streams on one channel may beincluded in, for example, a HE-SIG-A field. A HE-SIG-B field may includespatial stream allocation information about one subchannel. Also, aHE-SIG-C field may be transmitted after transmission of HE-LTFs,including Modulation and Coding Scheme (MCS) information about a PSDUand information about the length of the PSDU, etc.

FIG. 13 depicts OFDM symbol durations and GI lengths in the HE PPDUframe format according to the present disclosure.

In the HE PPDU frame format according to the present disclosure, L-STF,L-LTF, L-SIG, and HE-SIG-A fields may be configured with 4.0-μs OFDMsymbols based on 64-FFT. One OFDM symbol has a GI of 0.8 μs. In thepresent disclosure, a GI value applied to the L-STF, L-LTF, L-SIG, andHE-SIG-A fields is defined as G1. The L-STF, L-LTF, L-SIG, and HE-SIG-Afields may include 3.2-μs OFDM symbols based on 64-FFT, excluding theGIs. The term 64 FFT-based symbol is used mainly based on a channelbandwidth of 20 MHz. If the term 64 FFT-based symbol is usedirrespective of a channel bandwidth, a 64 FFT-based symbol may mean asymbol having a symbol duration of 3.2 μs and a subcarrier spacing of312.5 kHz.

The following HE-STF, HE-LTF, HE-SIG-B, and PSDU fields may include16-μs OFDM symbols based on 256-FFT. The OFDM symbol duration may bechanged according to a GI value. Two types of GI values may be definedfor one OFDM symbol during different time periods. A GI value applied tothe OFDM symbols of the HE-STF, HE-LTF, and HE-SIG-B fields is definedas G2 and a GI value applied to the OFDM symbols of the PSDU is definedas G3. Excluding the GIs, the HE-STF, HE-LTF, HE-SIG-B, and PSDU fieldsmay be configured with 12.8-μs OFDM symbols based on 256-FFT. The term256 FFT-based symbol is used mainly based on a channel bandwidth of 20MHz. If the term 256 FFT-based symbol is used irrespective of a channelbandwidth, a 256 FFT-based symbol may mean a symbol having a symbolduration of 12.8 μs and a subcarrier spacing of 78.125 kHz.

The values of G2 and G3 may be equal or different. If G2 and G3 areequal, G2 and G3 may be defined as one parameter without distinguishingbetween G2 and G3. Unlike G1, G2 and G3 may vary according to atransmitted PPDU transmission vector, rather than they are fixed values(i.e., predetermined values). This is related to that the lengths of theHE-STF, HE-LTF, and HE-SIG-B fields to which G2 is applied may varyaccording to a PPDU transmission vector and the length of the PSDU towhich G3 is applied may also vary according to the PPDU transmissionvector.

For example, G1 may have a fixed value (i.e., a predetermined value) of0.8 μs, G2 may be a value selected from 3.2 μs, 1.6 μs, 0.8 μs, and 0.4μs, and G3 may be a value selected from among 3.2 μs, 1.6 μs, 0.8 μs,and 0.4 μs. Also, G1 may have a fixed value (i.e., a predeterminedvalue) of 0.8 μs, and G2 or G3 may be a value selected or determinedfrom among 3.2 μs, 1.6 μs, 0.8 μs, and 0.4 μs. G1 does not requireseparate signaling because G1 is a fixed value, and signalinginformation indicating G2 and G3 may be provided in the HE-SIG-A fieldto a HE STA.

G2 and G3 are applied commonly across all OFDM symbols transmittedduring a corresponding time period and across all subchannels.Accordingly, PSDU transmission timings and OFDM symbol timings may besynchronized. For example, it is not allowed to apply a 3.2 μs G2 valueto a subchannel and a 1.6 μs or 0.8 μs G2 value to another subchannelduring a specific time period. Rather, the same 3.2-μs G2 value may beapplied to the subchannels during the same time period. In a similarexample, it is not allowed to apply a 1.6 μs G3 value to a subchanneland a 3.2 μs or 0.8 μs G3 value to another subchannel during a specifictime period. Rather, the same 1.6 μs G3 value may be applied to thesubchannels during the same time period.

In the case where a HE PPDU frame format having HE-LTF sections ofdifferent lengths for subchannels is used (i.e., in the case where thenumber of HE-LTFs is not determined for each subchannel based on themaximum of the numbers of HE-LTFs required according to the total numberof spatial streams transmitted on each subchannel in a set of HE STAsallocated to each of subchannels, as described in the example of FIG.11), if the values of G2 and G3 are different, PSDUs are transmitted ondifferent subchannels at different time points and OFDM symbol timingsare not synchronized. Therefore, values of G2 and G3 may need to beselected or determined as a same value.

In the case where a HE PPDU frame format having HE-LTF sections of thesame length for subchannels is used (i.e., in the case where the numberof HE-LTFs is determined for each subchannel based on the maximum of thenumbers of HE-LTFs required according to the total number of spatialstreams transmitted on each subchannel in a set of HE STAs allocated toeach of subchannels, as described in the example of FIG. 11), eventhough the values of G2 and G3 are different, PSDUs are transmitted onthe subchannels at the same time point, without causing discrepancybetween OFDM symbol timings. Therefore, values of G2 and G3 may beselected or determined as different values. However, even in this case,the present disclosure does not exclude that values of G2 and G3 may beselected or determined as a same value.

In the example of FIG. 13, OFDM symbol durations S1, S2, and S3 may beapplied respectively to time periods to which the GIs G1, G2, and G3 areapplied.

FIG. 14 depicts an exemplary HE PPDU frame format for a wide channelband according to the present disclosure.

Referring to FIG. 14, the HE PPDU frame format for one 20-MHz channelillustrated in the example of FIG. 13 is extended to two 20-MHzchannels. Similarly, HE PPDU frame formats for the channel bandwidths of80 MHz and 160 MHz may be configured by extending the HE PPDU frameformat for one 20-MHz channel illustrated in the example of FIG. 13 tofour and eight 20-MHz channels, respectively.

There is no modification involved in extending the HE PPDU frame formatfor one 20-MHz channel. In other words, all subchannels across one ormore 20-MHz channels are the same in terms of PSDU transmission timepoints, OFDM symbol durations, and GIs.

From this viewpoint, the example described with reference to FIG. 11 inwhich “the lengths of HE-LTF sections across subchannels are equal” maybe extended to simultaneous application on a channel basis as well as ona subchannel basis. Therefore, PSDU transmission timings and OFDM symboltimings are synchronized for users paired for MU-MIMO-mode or OFDMA-modetransmission, thus maintaining orthogonality. This channel-based examplewill be described below.

Basically in SU-MIMO-mode or non-OFDMA-mode transmission, it issufficient to transmit as many HE-LTFs as required. However, the timingsof fields transmitted on subchannels for other paired STAs need to besynchronized (or aligned) across all subchannels over one or more 20-MHzchannels in MU-MIMO-mode or OFDMA-mode transmission. Therefore, thenumbers of HE-LTFs on all other subchannels over one or more 20-MHzchannels may be determined based on a subchannel having a maximum numberof streams among all subchannels over one or more 20-MHz channels inMU-MIMO-mode or OFDMA-mode transmission.

Specifically, the numbers of HE-LTFs to be transmitted on allsubchannels may be determined according to the maximum of the numbers ofHE-LTFs required according to the total numbers of spatial streamstransmitted on each subchannels over one or more 20-MHz channels, for aset of HE STAs allocated to each of the subchannels. Herein, ‘the set ofHE STAs allocated to each of the subchannels over one or more 20-MHzchannels’ is one HE STA in the SU-MIMO mode, whereas it is a set of aplurality of HE STAs paired on all subchannels over one or more 20-MHzchannels in the MU-MIMO mode or OFDMA mode. The ‘total number of spatialstreams transmitted on each of all subchannels over one or more 20-MHzchannels’ is the number of spatial streams transmitted to one HE STA inthe SU-MIMO mode and the number of spatial streams transmitted to aplurality of HE STAs paired on the subchannel in the MU-MIMO mode orOFDMA mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point on all subchannels over one ormore 20-MHz channels for all users (i.e., HE STAs) in MU-MIMO-mode orOFDMA-mode transmission of a HE PPDU. Or it may be said that the lengthsof HE-LTF sections are equal on all subchannels over one or more 20-MHzchannels for all HE STAs in MU-MIMO-mode or OFDMA-mode transmission. Orit may be said that the number of HE-LTF elements included in eachHE-LTF section is equal in all subchannels over one or more 20-MHzchannels for all HE STAs in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized between allsubchannels over one or more 20-MHz channels for all HE STAs inMU-MIMO-mode or OFDMA-mode transmission.

In FIG. 14, the OFDM symbol duration and GI of L-STF, L-LTF, L-SIG, andHE-SIG-A fields on the first 20-MHz subchannel are S1 and G1,respectively. Like the first 20-MHz subchannel, the first 20-MHzsubchannel has S1 and G1 respectively as the OFDM symbol duration and GIof L-STF, L-LTF, L-SIG, and HE-SIG-A fields.

In FIG. 14, a HE-STF field, a plurality of HE-LTFs, and a HE-SIG-B fieldon the first 20-MHz channel are S2 and G2, respectively. Like the first20-MHz channel, the OFDM symbol duration and GI of a HE-STF field, aplurality of HE-LTFs, and a HE-SIG-B field on the second 20-MHz channelare also S2 and G2, respectively.

In FIG. 14, the OFDM symbol duration and GI of a PSDU on the first20-MHz channel are S3 and G3, respectively. Like the first 20-MHzchannel, the OFDM symbol duration and GI of a PSDU on the second 20-MHzchannel are also S3 and G3, respectively.

This example means that if the OFDM symbol duration and GI of one 20-MHzchannel are determined based on 64-FFT, the OFDM symbol duration and GIof the other 20-MHz channel(s) should be determined based on 64-FFT. Inother words, if the OFDM symbol duration and GI of one 20-MHz channelare determined based on 64-FFT, the OFDM symbol duration and GI of theother 20-MHz channel(s) should not be determined based on 256-FFT.

In a modification example, although subchannels within one 20-MHzchannel may have the same OFDM symbol durations and the same GIs,subchannels within another 20-MHz channel may have different OFDM symboldurations and GIs. For example, while S2, G2, S3, and G3 are applied asOFDM symbol durations and GIs for subchannels within the first 20-MHzchannel, different values (e.g., S4, G4, S5, and G5) may be applied asOFDM symbol durations and GIs for subchannels within the second 20-MHzchannel. Even in this case, the OFDM symbol duration and GI, S1 and G1,applied to L-STF, L-LTF, and L-SIG fields in a different 20-MHz channelare the same fixed values in every 20-MHz channel.

Further, this modification example may include application of theexample described before with reference to FIG. 11 in which ‘subchannelshave the same HE-LTF section length’ only to subchannels within one20-MHz channel, not to the HE-LTF section length of subchannels inanother 20-MHz channel.

With reference to the foregoing examples of the present disclosure,mainly the features of a HE PPDU frame structure applicable to a DLMU-MIMO-mode or OFDMA-mode transmission that an AP transmitssimultaneously to a plurality of STAs have been described. Now, adescription will be given of the features of a HE PPDU frame structureapplicable to a UL MU-MIMO-mode or OFDMA-mode transmission that aplurality of STAs transmits simultaneously to an AP.

The above-described various examples of structures of the HE PPDU frameformat supporting MU-MIMO-mode or OFDMA-mode transmission should not beunderstood as applicable only to DL without applicable UL. Rather, theexamples should be understood as also applicable to UL. For example, theabove-described exemplary HE PPDU frame formats may also be used for aUL HE PPDU transmission that a plurality of STAs simultaneouslytransmits to a single AP.

However, in the case of a DL MU-MIMO-mode or OFDMA-mode HE PPDUtransmission that an AP simultaneously transmits to a plurality of STAs,the transmission entity, AP has knowledge of the number of spatialstreams transmitted to a HE STA allocated to each of a plurality ofsubchannels. Therefore, the AP may include, in a HE-SIG-A field or aHE-SIG-B field, information about the total number of spatial streamstransmitted across a channel, a maximum number of spatial streams (i.e.,information being a basis of the number of HE-LTF elements (or thestarting point and ending point of a HE-LTF section) on eachsubchannel), and the number of spatial streams transmitted on eachsubchannel. In contrast, in the case of a UL MU-MIMO-mode or OFDMA-modeHE PPDU transmission that a plurality of STAs simultaneously transmitsto an AP, each STA being a transmission entity may be aware only of thenumber of spatial streams in a HE PSDU that it will transmit, withoutknowledge of the number of spatial streams in a HE PSDU transmitted byanother STA paired with the STA. Accordingly, the STA may determineneither the total number of spatial streams transmitted across a channelnor a maximum number of spatial streams.

To solve this problem, a common parameter (i.e., a parameter appliedcommonly to STAs) and an individual parameter (a separate parameterapplied to an individual STA) may be configured as follows in relationto a UL HE PPDU transmission.

For simultaneous UL HE PPDU transmissions from a plurality of STAs to anAP, a protocol may be designed in such a manner that the AP sets acommon parameter or individual parameters (common/individual parameters)for the STAs for the UL HE PPDU transmissions and each STA operatesaccording to the common/individual parameters. For example, the AP maytransmit a trigger frame (or polling frame) for a UL MU-MIMO-mode orOFDMA-mode transmission to a plurality of STAs. The trigger frame mayinclude a common parameter (e.g., the number of spatial streams across achannel or a maximum number of spatial streams) and individualparameters (e.g., the number of spatial streams allocated to eachsubchannel), for the UL MU-MIMO-mode or OFDMA-mode transmission. As aconsequence, a HE PPDU frame format applicable to a UL MU-MIMO or OFDMAmode may be configured without a modification to an exemplary HE PPDUframe format applied to a DL MU-MIMO or OFDMA mode. For example, eachSTA may configure a HE PPDU frame format by including information aboutthe number of spatial streams across a channel in a HE-SIG-A field,determining the number of HE-LTF elements (or the starting point andending point of a HE-LTE section) on each subchannel according to themaximum number of spatial streams, and including information about thenumber of spatial streams for the individual STA in a HE-SIG-B field.

Alternatively, if the STAs operate always according to thecommon/individual parameters received in the trigger frame from the AP,each STA does not need to indicate the common/individual parameters tothe AP during a HE PPDU transmission. Therefore, this information maynot be included in a HE PPDU. For example, each STA may have only todetermine the total number of spatial streams, the maximum number ofspatial streams, and the number of spatial streams allocated toindividual STA, as indicated by the AP, and configure a HE PPDUaccording to the determined numbers, without including information aboutthe total number of spatial streams or the number of spatial streamsallocated to the STA in the HE PPDU.

On the other hand, if the AP does not provide common/individualparameters in a trigger frame, for a UL MIMO-mode or OFDMA-mode HE PPDUtransmission, the following operation may be performed.

Common transmission parameters (e.g., channel BandWidth (BW)information, etc.) for simultaneously transmitted HE PSDUs may beincluded in HE-SIG-A field, but parameters that may be different forindividual STAs (e.g., the number of spatial streams, an MCS, andwhether STBC is used or not, for each individual STA) may not beincluded in HE-SIG-A field. Although the individual parameters may beincluded in HE-SIG-B field, information about the number of spatialstreams and information indicating whether STBC is used or not, need tobe transmitted before a HE-LTF field because the number of spatialstreams and the information indicating whether STBC is used or not aresignificant to determination of configuration information about apreamble and a PSDU in a HE PPDU frame format (e.g., the number ofHE-LTF elements is determined according to a combination of the numberof spatial streams and the information indicating whether STBC is usedor not). For this purpose, a HE PPDU frame format as illustrated in FIG.15 may be used for a UL HE PPDU transmission.

FIG. 15 depicts another exemplary HE PPDU frame format according to thepresent disclosure. The HE PPDU frame format illustrated in FIG. 15 ischaracterized in that a structure of HE-SIG-A, HE-SIG-B, and HE-SIG-Cfields similar to in FIG. 11 is used for a UL PPDU transmission.

As described before, if a UL MU-MIMO-mode or OFDMA-mode transmission isperformed by triggering of an AP (according to common/individualparameters provided by the AP), an individual STA may not need to reportan individual parameter to the AP. In this case, one or more of aHE-SIG-B field, a HE-SIG-C field, and a first HE-LTF element (i.e., aHE-LTF between a HE-STF field and a HE-SIG-B field) illustrated in FIG.15 may not exist. In this case, a description of each field given belowmay be applied only in the presence of the field.

In the example of FIG. 15, a HE-SIG-A field is transmitted per channel(i.e., per 20-MHz channel) and may include transmission parameterscommon to simultaneously transmitted HE PSDUs. Since the sameinformation is transmitted in up to HE-SIG-A fields in UL PPDUstransmitted by HE STAs allocated to subchannels, the AP may receive thesame signals from the plurality of STAs successfully.

A HE-SIG-B field is transmitted per subchannel in one channel. TheHE-SIG-B field may have an independent parameter value according to thetransmission characteristics of a HE PSDU transmitted on eachsubchannel. The HE-SIG-B field may include spatial stream allocationinformation and information indicating whether STBC is used or not, foreach subchannel. If MU-MIMO is applied to a subchannel (i.e., if aplurality of STAs perform transmission on a subchannel), the HE-SIG-Bfield may include a common parameter for the plurality of STAs paired onthe subchannel.

A HE-SIG-C field is transmitted on the same subchannel as the HE-SIG-Bfield and may include information about an MCS and a packet length. IfMU-MIMO is applied to a subchannel (i.e., if a plurality of STAs performtransmission on a subchannel), the HE-SIG-C field may include respectiveindividual parameters for each of the plurality of STAs paired on thesubchannel.

Similarly to DL MU-MIMO-mode or OFDMA-mode HE PPDU transmission,transmissions of PSDUs may start at different time points on subchannelsin UL MU-MIMO-mode or OFDMA-mode HE PPDU transmission, and if OFDMsymbols are not aligned accordingly, then the implementation complexityof an AP that receives a plurality of PSDUs increased. To solve thisproblem, ‘the number of HE-LTFs may be determined for all subchannelsaccording to the maximum of the numbers of HE LTFs required according tothe total numbers of spatial streams transmitted on each subchannel fora set of HE STAs allocated to each of subchannels’ as described withreference to the example of FIG. 11.

This feature may mean that the HE-LTF field start at the same time pointand end at the same time point across all users (i.e., HE STAs) in ULMU-MIMO-mode or OFDMA-mode transmission. Or it may be said that theHE-LTF sections of a plurality of subchannels have the same lengthacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission. Or itmay be said that each of the HE-LTF sections of a plurality ofsubchannels includes the same number of HE-LTF elements across all HESTAs in UL MU-MIMO-mode or OFDMA-mode transmission. Therefore, PSDUtransmission timings are synchronized between a plurality of subchannelsacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission.

In the HE PPDU frame format supporting UL MIMO-mode or OFDMA-modetransmission illustrated in FIG. 15, the L-STF, L-LTF, L-SIG, andHE-SIG-A fields may include 4.0-μs OFDM symbols based on 64-FFT. OneOFDM symbol has a GI of 0.8 μs. In the present description, A GI valueapplied to the L-STF, L-LTF, L-SIG, and HE-SIG-A fields is defined asG1. Excluding the GI, the L-STF, L-LTF, L-SIG, and HE-SIG-A fields maybe configured as 3.2-μs OFDM symbols based on 64-FFT.

In the example of FIG. 15, a HE-STF field, a HE-LTF field, a HE-SIG-Bfield, HE-LTF elements(s) in a HE-LTF section, HE-SIG-C field and a PSDUmay include 16-μs OFDM symbols based on 256-FFT. The OFDM symbolduration may be changed according to a GI value. Two types of GI valuesmay be defined for one OFDM symbol for different time periods. A GIvalue applied to the OFDM symbols of the HE-STF field, the HE-LTF field,the HE-SIG-B field, the HE-LTF elements(s) in the HE-LTF section, andthe HE-SIG-C field is defined as G2 and a GI value applied to the OFDMsymbols of the PSDU is defined as G3. Excluding the GIs, the HE-STFfield, the HE-LTF field, the HE-SIG-B field, and the PSDU may include12.8-μs OFDM symbols based on 256-FFT.

The values of G2 and G3 may be equal or different. If G2 and G3 areequal, G2 and G3 may be defined as one parameter without distinguishingG2 from G3. Unlike G1, G2 and G3 may vary according to an eachtransmitted PPDU transmission vector, rather than they are fixed values(i.e. predetermined values known to both a transmitter and a receiver).This is related to the fact that the lengths of the HE-STF, the HE-LTF,the HE-SIG-B, the HE-LTF element(s) in a HE-LTF section, and theHE-SIG-C to which G2 is applied may vary according to a PPDUtransmission vector and the length of the PSDU to which G3 is appliedmay also vary according to the PPDU transmission vector.

In another example, the G1 applied to the L-STF, L-LTF, L-SIG, andHE-SIG-A fields (to which 64-FFT is applied) may be a fixed value (i.e.,a predefined value known to both a transmitter and a receiver) and oneof G2 and G3 (if G2 and G3 are equal, they may be defined as oneparameter) applied to the following fields (i.e., the HE-STF, HE-LTF,HE-SIG-B, HE-SIG-C, and PSDU to which 256-FFT is applied) may beconfigured or indicated as a variable value (e.g., one of 3.2 μs, 1.6μs, 0.8 μs, and 0.4 μs).

More specifically, G1 may have a fixed value (i.e. a predefined valueknown to both a transmitter and a receiver) of 0.8 μs, G2 may be a valueselected or indicated from among 3.2 μs, 1.6 μs, 0.8 μs, and 0.4 μs, andG3 may be a value selected or indicated from among 3.2 μs, 1.6 μs, 0.8μs, and 0.4 μs. Also, G1 may be a fixed value (i.e. a predefined valueknown to both a transmitter and a receiver) of 0.8 μs, and G2 or G3 maybe a value selected or indicated from among 3.2 μs, 1.6 μs 0.8 μs and0.4 μs. G1 does not require signaling because G1 is a fixed value, andsignaling information indicating G2 and G3 may be provided to the AP. Ifa HE STA performs UL transmission according to triggering of the AP (orbased on parameters provided by the AP), the HE-STA does not need toindicate the value of G2 or G3 to the AP.

G2 and G3 are applied commonly across all OFDM symbols transmittedduring a corresponding time period and across all subchannels.Accordingly, PSDU transmission timings may be synchronized, and OFDMsymbol timings may be synchronized. For example, it is not allowed that,if a 3.2 μs G2 value is applied to a subchannel during a specific timeperiod, a 1.6 μs or 0.8 μs G2 value is applied to other subchannelsduring the same time period. Rather, the same 3.2-μs G2 value may beapplied to other subchannels during the same time period. In a similarexample, it is not allowed that, if a 1.6 μs G3 value is applied to asubchannel during a specific time period, a 3.2 μs or 0.8 μs G3 value isapplied to other subchannels during the same time period. Rather, thesame 1.6-μs G3 value may be applied to other subchannels during the sametime period.

In the case where a HE PPDU frame format having HE-LTF sections ofdifferent lengths for subchannels is used (i.e., in the case where ‘thenumber of HE-LTFs is not determined for each subchannel based on themaximum of the numbers of HE-LTFs required according to the totalnumbers of spatial streams transmitted on subchannels in a set of HESTAs allocated to each of the subchannels’), if the values of G2 and G3are different, a PSDU is transmitted on each subchannel at a differenttime point and OFDM symbol timings are not synchronized. Therefore, thesame values may need to be selected or indicated as G2 and G3 in thiscase.

In the case where a HE PPDU frame format having HE-LTF sections of thesame length for subchannels is used (i.e., in the case where ‘the numberof HE-LTFs is determined for each subchannel based on the maximum of thenumbers of HE-LTFs required according to the total numbers of spatialstreams transmitted on subchannels in a set of HE STAs allocated to eachof the subchannels’), even though the values of G2 and G3 are different,PSDUs are transmitted on the subchannels at the same time point, withoutcausing discrepancy between OFDM symbol timings. Therefore, selection orindication of different values as G2 and G3 does not cause a problem.However, even in this case, selection or indication of the same valuesas G2 and G3 is not excluded.

In the example of FIG. 15, OFDM symbol durations S1, S2, and S3 may beapplied respectively to time periods to which the GIs G1, G2, and G3 areapplied.

As described before, a plurality of STAs may simultaneously transmitPSDUs in a HE PPDU frame format on their allocated subchannels or ontheir allocated spatial streams to an AP (i.e., referred to as ULMU-MIMO or OFDMA transmission or “UL MU transmission”) and maysimultaneously receive PSDUs in the HE PPDU frame format on theirallocated subchannels on their allocated spatial streams from the AP(i.e., referred to as DL MU-MIMO or OFDMA transmission or “DL MUtransmission”).

Hereinafter, examples of the present disclosure for applying beamformingand acquiring Channel State Information (CSI) in HE PPDU transmissionfor DL/UL MU transmission will be described. Beamforming transmissionmay mean a transmission scheme for controlling directions of beams toeach STA and the CSI may include channel state information needed tocalculate a beamforming steering matrix. Specifically, a descriptionwill be given of a HE PPDU frame configuration when beamforming isapplied for DL/UL MU transmission, a procedure for a beamformer (i.e., atransmitter of a beamformed signal) to acquire the CSI from a beamformee(i.e., a receiver of the beamformed signal), and a frame configurationused for the CSI acquisition procedure.

According to the present disclosure, in DL MU transmission or UL MUtransmission, whether beamforming is applied to a plurality ofsubchannels (or a plurality of STAs) in a HE PPDU transmitted over onetransmission channel divided into the plurality of subchannels may beidentically or differently configured.

In addition, information indicating whether beamforming is applied toeach subchannel (or each STA) may be included in a HE-SIG field in a HEPPDU format supporting beamforming in DL MU transmission or UL MUtransmission. Further, information directly or indirectly indicatingwhether beamforming is applied to each subchannel (or each STA) may beincluded in a trigger frame triggering UL MU transmission in order tosupport beamforming in UL MU transmission.

According to the present disclosure, an AP may simultaneously receiveCSI from a plurality of STAs for beamformed DL MU transmission. For theAP to acquire the CSI from the multiple STAs, an NDP sounding proceduremay be performed. The AP may determine whether beamforming is applied tothe multiple STAs (or multiple subchannels allocated to the multipleSTAs) based on the CSI acquired from the multiple STAs.

Hereinafter, various embodiments of the present disclosure forbeamforming support in a HE PPDU will be described in more detail.

FIGS. 16 and 17 depict exemplary HE PPDU frame formats supportingbeamforming according to the present disclosure.

In the example of FIG. 16, an AP corresponding to a beamformer transmitsPSDUs over subchannels allocated respectively to STA1, STA2, STA3, andSTA4 corresponding to beamformees. Signaling information indicatingwhether beamforming is applied to a PSDU over each subchannel may beincluded in a HE-SIG-B field of a HE PPDU.

Whether beamforming is applied to each of multiple subchannels in onechannel may be individually or independently determined That is,beamforming may be identically or differently applied to multiplesubchannels (or multiple STAs). In this case, signaling bits having thesame size as the number of subchannels are needed. For example, whetherbeamforming is applied to each of N subchannels may be indicated byN-bit signaling information and a bit value (i.e., 0 or 1) of a bitposition of each of N bits may indicate that beamforming is not appliedor is applied to one subchannel.

Alternatively, whether beamforming is applied to a plurality ofsubchannels in one channel may be commonly determined That is,beamforming may be identically applied or may not be identically appliedto all the subchannels. In this case, the signaling information may havea size of one bit.

In the example of FIG. 17, an AP corresponding to a beamformer transmitsPSDUs over subchannels allocated respectively to STA1, STA2, STA3, andSTA4 corresponding to beamformees. Signaling information indicatingwhether beamforming is allocated to a PSDU over each subchannel may beincluded in a HE-SIG-B field of a HE PPDU.

If the HE-SIG-B field is transmitted over each subchannel, 1-bitsignaling information indicating whether beamforming is applied to acorresponding subchannel may be included in one HE-SIG-B field. Whetherbeamforming is applied to each of multiple subchannels may beindividually or independently determined.

If the HE-SIG-B field is commonly transmitted over the multiplesubchannels, N-bit signaling information individually indicating whetherbeamforming is applied to N subchannels or 1-bit signaling informationcommonly indicating whether beamforming is applied to the N subchannelsmay be included in the HE-SIG-B field.

In this way, the signaling information indicating whether beamforming isapplied to the multiple subchannels may be included in a HE-SIG field(e.g., the HE-SIG-A or HE-SIG-B field). More specifically, the signalinginformation indicating whether beamforming is applied to the multiplesubchannels may be included in the HE-SIG field transmitted prior to aHE-LTF field. A HE-LTF section in each subchannel is used for channelestimation of the corresponding subchannel and channel estimation mayvary according to whether beamforming is applied to the subchannel.Therefore, the information indicating whether beamforming is applied maybe transmitted at least before the HE-LTF field (or before a HE-STFfield).

In addition, in beamformed HE PPDU transmission, a beamforming steeringmatrix may be applied to a transmission signal waveform and, in thiscase, smoothing may not be applied. Smoothing refers to acquisition ofnoise suppression gain in consideration of a neighboring subcarriersignal in channel estimation, using a high channel correlation betweenadjacent subcarriers in a channel environment in which delay spread islow. In beamformed HE PPDU transmission, since a channel correlationbetween adjacent subcarriers may not be maintained, channel estimationperformed by reflecting an adjacent subcarrier signal through smoothingmay not necessarily ensure noise suppression. Specifically, if whetherbeamforming is applied to each of multiple subchannels in beamformed HEPPDU transmission is individually or independently determined,subchannels to which beamforming is not applied and subchannels to whichbeamforming is applied may be adjacent to each other and, in this case,smoothing may not be applied to adjacent subcarriers over a subchannelboundary.

Thus, a certain STA may not apply smoothing to subcarrier(s) located ata boundary of subcarriers allocated thereto. For example, in the exampleof FIG. 16 or 17, if STA2 is not aware of whether beamforming is appliedto other subchannels (e.g., a subchannel allocated to STA1 and asubchannel allocated to STA3) adjacent to a subchannel allocated toSTA2, STA2 may not apply smoothing to subcarriers located at asubcarrier boundary (i.e., subcarriers adjacent to the subchannelallocated to STA1 and subcarriers adjacent to the subchannel allocatedto STA3) in the subchannel allocated to STA2.

FIG. 18 depicts an exemplary sounding protocol according to the presentdisclosure.

A sounding protocol is a procedure in which an AP acquires feedbackinformation about CSI between the AP and an STA from the STA in orderfor the AP to perform beamforming transmission to the STA. Through thesounding procedure, the AP may transmit an NDP frame (i.e., a PPDU framehaving a PSDU of length 0) to the STA and the STA may determine the CSIbased on the NDP frame and transmit a determined result (i.e., feedbackinformation) to the AP. The CSI determined based on the NDP frame may befed back to the AP that has requested sounding through a compressedbeamforming frame. The NDP frame transmitted by the AP may be receivedby a plurality of STAs and the AP may acquire the feedback informationfrom each STA.

Prior to transmission of the NDP frame by the AP, an NDP Announcement(NDPA) frame may be transmitted. The NDPA frame is a PPDU frameindicating that the NDP frame immediately follows the NDPA frame (e.g.,that the NDP frame will be transmitted after an SIFS from an end timepoint of the NDPA frame). The first STA of an STA list included in theNDPA frame may receive the NDP frame even without receiving anadditional polling frame (e.g. a Beamforming Report Polling frame) fromthe AP and then immediately transmit a feedback frame to the AP (e.g.,after an SIFS from a reception end time point of the NDP frame). Theother STAs included in the NDPA frame may receive the polling frame(e.g., the Beamforming Report Poll frame) from the AP and thenimmediately transmit the feedback frame (e.g., a frame including aBeamforming Report field) to the AP (e.g., after an SIFS from areception end time point of the polling frame). Herein, the pollingframe may be transmitted from the AP to an STA immediately after acompressed beamforming frame from the STA is ended (e.g., after an SIFSfrom an end time point of the compressed beamforming frame).

The NDPA frame should be transmitted so as to be received by target STAsof the sounding procedure. Therefore, beamforming may not be applied totransmission of the NDPA frame (i.e. the NDPA frame may beomnidirectionally transmitted).

In this way, since both the NDP frame and the Beamforming Report Pollframe, transmitted by an STA (e.g., an AP) requesting feedback in thesounding procedure, have features in common in that they are frames fortriggering the feedback frame (e.g., a frame including a BeamformingReport field) from target STAs (e.g., STA1, STA2, STA3, and STA4) of thesounding procedure, the NDPA frame, the NDP frame and the BeamformingReport Poll frame may be collectively referred to as feedback triggerframes.

In the example of FIG. 18, an AP corresponding to a beamformer transmitsthe NDPA frame to STA1, STA2, STA3, and STA4 corresponding tobeamformees and transmits the NDP frame (i.e. the PPDU frame onlyincluding a legacy preamble (i.e., an L-STF, an L-LTF, and L-SIG field)and a HE-preamble (i.e. a HE-SIG-A field, a HE-STF, and HE-LTF)) afteran SIFS. STA1, STA2, STA3, and STA4 corresponding to the beamformees maybe aware that they are target STAs of NDP sounding by receiving the NDPAframe and the NDP frame and perform channel estimation using the NDPframe. After receiving the feedback trigger frame (e.g., the NDPA frame,the NDP frame, or the Beamforming Report Poll frame), each STA transmitsthe result (e.g., CSI) of channel estimation to the AP through a HEcompressed beamforming frame after an SIFS.

In the example of FIG. 18, a plurality of STAs simultaneously transmitsHE compressed beamforming frames (e.g., frames including the CSI).

Specifically, in the example of FIG. 18, it is assumed that beamformeeSpace Time Stream (STS) capabilities of STA1 and STA2 indicate that amaximum of two STSs is supported and beamformee STS capabilities of STA3and STA4 indicate that a maximum of four STSs is supported. In thiscase, the NDP frame cannot request channel estimation for STSs exceedingthe number of beamformee STS capabilities. For example, if beamformeeSTS capabilities indicate that a maximum of two STSs is supported, theNDP frame transmitted by the beamformer may request channel estimationfor the two STSs and cannot request channel estimation for more than twoSTSs (e.g., 3, 4, 5, 6, 7, or 8 STSs).

Since the number of HE-LTFs in the HE PPDU is determined based on thenumber of STSs as described above, the number of HE-LTFs used forchannel estimation for STSs in the NDP frame may be determined based onthe beamformee STS capabilities. That is, as in the example of FIG. 18,the NDP frame for STA1 and STA2 supporting a maximum of two STSs mayinclude two HE-LTFs and the NDP frame for STA3 and STA4 supporting amaximum of four STSs may include four HE-LTFs.

Furthermore, in NDP sounding according to the present disclosure,simultaneously performing NDP sounding with respect to STAs havingdifferent beamformee STS capabilities may be supported. As in theexample of FIG. 18, NDP sounding over a low channel (e.g., one 20 MHzchannel of a low frequency side out of two 20 MHz channels constitutinga 40 MHz channel when NDP sounding is performed over a 40 MHz channel inFIG. 18) for STA1 and STA2, beamformee STS capabilities of which supporttwo STSs, and NDP sounding over a high channel (e.g., one 20 MHz channelof a high frequency side out of two 20 MHz channels constituting a 40MHz channel) for STA3 and STA4, beamformee STS capabilities of whichsupport four STSs, may be simultaneously performed.

Specifically, in the example of FIG. 18, the AP may request that STAstransmit CSI on a channel (or subchannel) basis and receive the CSI. Forexample, the AP may perform NDP sounding using a 40 MHz channel withrespect to STA1, STA2, STA3, and STA4. In this case, the AP may transmitthe NDPA frame and the NDP frame over a low 20 MHz channel for thepurpose of requesting that STA1 and STA2 transmit the CSI for a low 20MHz channel and then the AP may receive the CSI of a low 20 MHz channelthrough the compressed beamforming frame from STA1 and STA2. At the sametime, the AP may transmit the NDPA frame and the NDP frame over a high20 MHz channel for the purpose of requesting that STA3 and STA4 transmitCSI for a high 20 MHz channel and then the AP may receive the CSI of ahigh 20 MHz channel through the compressed beamforming frame from STA3and STA4.

The NDPA frame transmitted over a low channel may include STA1 and STA2in an NDP sounding target STA list and the NDPA frame transmitted over ahigh channel may include STA3 and STA4 in the NDP sounding target STAlist. The NDPA frames over a low channel and a high channel may besimultaneously transmitted.

The NDP frame transmitted after an SIFS from an NDPA frame end timepoint over a low channel may include two HE-LTFs for channel estimationfor two STSs corresponding to beamformee STS capabilities of STA1 andSTA2. The NDP frame transmitted after an SIFS from an NDPA frame endtime point over a high channel may include four HE-LTFs for channelestimation for four STSs corresponding to beamformee STS capabilities ofSTA3 and STA4. The NDP frames over a low channel and a high channel maybe simultaneously transmitted.

If the number of HE-LTFs of the NDP frame transmitted over a low channelis different from that of the NDP transmitted over a high channel,transmission times of the NDP frames (i.e. NDP frame end time points)may be different. In this case, HE compressed beamforming framestransmitted as an immediate response to the NDP frames may becomedifferent over a low channel and a high channel.

To simultaneously transmit the HE compressed beamforming frames over alow channel and a high channel, the transmission times of the NDP frames(i.e., lengths of the NDP frames) over a plurality of channels may beset to be equal. To equally match the length of the NDP frametransmitted over a low channel with the length of the NDP frametransmitted over a high channel as in the example of FIG. 18, a paddingfield may be included in the NDP frame transmitted over a low channelafter a HE preamble. The padding field may correspond to additionaltransmission of a field of the HE preamble (e.g., a HE-LTF field or aHE-SIG-B field) or may be configured by a signal having a predeterminedpattern. If the padding field is added to any NDP frame, the length ofthe padding field may be set to a length difference between any NDPframe and another simultaneously transmitted NDP frame (e.g., differencebetween the length of the NDP frame transmitted over a high channel andthe length of the NDP frame transmitted over a low channel).

FIG. 19 depicts an exemplary NDPA frame format according to the presentdisclosure.

In the example of FIG. 19, a frame format of an MPDU included in a PSDUof a PPDU (or a HE PPDU) corresponding to an NDPA is illustrated.

A Receiver Address (RA) field may be set to an address of a receivingSTA of a corresponding frame. If an NDPA frame includes one STAInformation (Info) field, the RA field may be set to an address of acorresponding STA. If the NDPA frame includes a plurality of STA Infofields, the RA field may be set to a broadcast address.

A Transmitter Address (TA) field may be set to an address of atransmitting STA of a corresponding frame.

A Sounding Dialog Token field may be set to a value selected by abeamformer (e.g., an AP) in order to identify the NDPA frame.

The STA Info field may include information for designating beamformee(s)corresponding to a target STA of NDP sounding. The NDPA frame mayinclude one or multiple STA Info fields.

One STA Info field may include at least one of an Association Identifier(AID) subfield, a Feedback Type subfield, an Nc Index subfield, an NrIndex subfield, a Grouping subfield, and a Codebook Informationsubfield.

The AID subfield (e.g., 12-bit size) may include 12 Least SignificantBits (LSBs) of an AID of an STA that is expected to prepare soundingfeedback by processing an NDP frame following the NDPA frame. If the STAis an AP, a mesh STA, or a member of an IBSS, the AID subfield may beset to a value of 0.

The Feedback Type subfield (e.g., 1-bit size) may indicate a requestedfeedback type. If the Feedback Type subfield is set to 0, this mayindicate SU type feedback and, if the Feedback Type subfield is set to1, this may indicate MU type feedback.

The Nc Index subfield (e.g., 3-bit size) may be set to a value obtainedby subtracting one from the number of columns of a CompressedBeamforming Feedback Matrix subfield. If Nc=1, 2, 3, . . . , 8, then theNc Index subfield may be set to a value of 0, 1, 2, . . . , 7,respectively.

Herein, the Compressed Beamforming Feedback Matrix subfield may beincluded in a Compressed Beamforming Report field of a HE CompressedBeamforming frame and may include information about a compressedbeamforming feedback matrix to be applied to a specific subcarrier. Thecompressed beamforming feedback matrix may have a plurality of rows andone or more columns and elements of the matrix may indicate respectivespecific angles. The compressed beamforming feedback matrix fed backthrough the Compressed Beamforming Report field may be used for atransmission beamformer to determine a steering matrix.

The Nr Index subfield (e.g., 3-bit size) may be set to a value obtainedby subtracting one from the number of rows of the compressed beamformingfeedback matrix. If Nr=1, 2, 3, . . . , 8, then the Nr Index subfieldmay be set to a value of 0, 1, 2, . . . , 7, respectively.

The Grouping subfield (e.g., 2-bit size) may be set to a valueindicating a subcarrier grouping level, i.e., Ng, used for thecompressed beamforming feedback matrix. If Ng=1 (i.e., if there is nosubcarrier grouping), the Grouping subfield may be set to a value of 0.If Ng=2 (i.e., if only one compressed beamforming feedback matrix isreported with respect to a group including two adjacent subcarriers),the Grouping subfield may be set to a value of 1. If Ng=4 (i.e., if onlyone compressed beamforming feedback matrix is reported with respect to agroup including four adjacent subcarriers), the Grouping subfield may beset to a value of 2. The Grouping subfield set to a value of 3 may bereserved.

The Codebook Information subfield (e.g., 1-bit size) may be set to avalue indicating the size of a codebook entry.

Upon receiving the NDPA frame and the NDP frame following the NDPAframe, configured as described above, beamformees (e.g., STAs) maytransmit a plurality of HE compressed beamforming frames to a beamformer(e.g., AP) over channels receiving the NDPA and the NDP.

Herein, the multiple HE compressed beamforming frames transmitted by themultiple beamformees (e.g., multiple HE compressed beamforming frames asan immediate response to the NDP frame or multiple HE compressedbeamforming frames transmitted as an immediate response to theBeamforming Report Poll frame) may be simultaneously transmitted basedon configuration by the beamformer. That is, the multiple beamformeesmay simultaneously transmit the multiple HE compressed beamformingframes based on the NDPA frame transmitted from the beamformer to themultiple beamformees. For example, the multiple beamformees maysimultaneously transmit the multiple HE compressed beamforming framesbased on information (e.g., one or more of the Feedback Type, Nc Index,Nr Index, Grouping, and Codebook Information subfields of the STA Infofield) included in the NDPA frame transmitted from the beamformer to themultiple beamformees.

To simultaneously transmit the multiple HE compressed beamforming framesby the multiple beamformees, the sizes of the HE compressed beamformingframes transmitted by the multiple beamformees may be identically set.The sizes of the HE compressed beamforming frames may be determinedbased on the information included in the NDPA frame (e.g., one or moreof the Feedback Type, Nc Index, Nr Index, Grouping, and CodebookInformation subfields of the STA Info field). Specifically, each pieceof information included in the NDPA frame provided to the multiplebeamformees by the beamformer (e.g., one or more of the Feedback Type,Nc Index, Nr Index, Grouping, and Codebook Information subfields of theSTA Info field) may be set to the same value with respect to themultiple beamformees that simultaneously transmit the multiplecompressed beamforming frames. Referring back to FIG. 18, the sizes ofthe HE compressed beamforming frames of STA1 and STA2 may be equally setand the sizes of the HE compressed beamforming frames of STA3 and STA4may be equally set.

FIG. 20 depicts another exemplary sounding protocol according to thepresent disclosure.

FIG. 20 illustrates an additional example for simultaneously performingNDP sounding with respect to STAs having different beamformee STScapabilities. To this end, a plurality of durations corresponding todifferent beamformee STS capabilities may be consecutively transmittedin a transmission duration of a HE-LTE field used for channel estimationfor an STS in an NDP frame format.

In the example of FIG. 20, it may be assumed that beamformee STScapabilities of STA1 and STA2 are two STSs and beamformee STScapabilities of STA3 and STA4 are four STSs. In this case, the number ofHE-LTFs out of a HE preamble transmitted after a legacy preamble (anL-STF, an L-LTF, and an L-SIG field) in an NDP frame may be a total of6. That is, the HE-LTFs of the NDP frame may include HE-LTFs adding twoHE-LTFs for channel estimation of two STSs for STA1 and STA2 and fourHE-LTFs for channel estimation of four STSs for STA3 and STA4.

In this case, information indicating which HE-LTF is used for channelestimation of a certain STA (or an STS of which STA) may be included inthe NDPA frame.

As such, STA1 or STA2 may transmit a HE compressed beamforming frameincluding a channel estimation value (e.g., CSI) calculated based on thefirst two HE-LTFs in a HE preamble of the NDP frame to the AP as animmediate response to the NDP frame or an immediate response to aBeamforming Report Poll frame. STA3 or STA4 may transmit a HE compressedbeamforming frame including a channel estimation value (e.g., CSI)calculated based on the next four HE-LTFs of the first two HE-LTFs inthe HE preamble of the NDP frame to the AP as an immediate response tothe NDP frame or as an immediate response to the Beamforming Report Pollframe.

FIG. 21 depicts another exemplary sounding protocol according to thepresent disclosure.

FIG. 21 illustrates an additional example for simultaneously performingNDP sounding with respect to STAs having different beamformee STScapabilities. To this end, two different NDP frames may be transmittedat an interval of an SIFS after NDPA frame transmission.

In the example of FIG. 21, it may be assumed that beamformee STScapabilities of STA1 and STA2 are two STSs and beamformee STScapabilities of STA3 and STA4 are four STSs. In this case, two HE-LTFsfor channel estimation of two STSs for STA1 and STA2 may be included inthe first transmitted NDP frame and four HE-LTFs for channel estimationof four STSs for STA3 and STA4 may be included in the second transmittedNDP frame.

In this case, information indicating which NDP frame is used for channelestimation of a certain STA (or an STS of which STA) may be included inthe NDPA frame.

As such, STA1 or STA2 may transmit a HE compressed beamforming includinga channel estimation value (e.g., CSI) calculated based on two HE-LTFsof the first NDP frame to the AP as an immediate response to the lastNDP frame or an immediate response to the Beamforming Report Poll frame.STA3 or STA4 may transmit a HE compressed beamforming frame including achannel estimation value (e.g., CSI) calculated based on four HE-LTFs ofthe second NDP frame to the AP as an immediate response to the NDP frameor as an immediate response to the Beamforming Report Poll frame.

Although the NDPA frame is transmitted to STA1 and STA3, and STA1 andSTA3 transmit the HE compressed beamforming frames in the illustratedexamples of FIGS. 20 and 21, the present disclosure is not limitedthereto. That is, the NDPA frame for designating one or more of STA1,STA2, STA3, and STA4 as NDP sounding target STAs may be transmitted fromthe AP, one NDP frame (i.e. the example of FIG. 20) or multiple NDPframes (i.e. the example of FIG. 21) for channel estimation for thedesignated NDP sounding target STAs may be transmitted from the AP, andthe HE compressed beamforming frame from each of the designated targetSTAs may be transmitted to the AP as an immediate response to the NDPframe or as an immediate response to the Beamforming Report Poll frame.

UL MU-MIMO transmission may be taken as an example of UL MU transmissionin the following examples of the present disclosure. However, theexamples of the present disclosure are also applicable in the samemanner to UL OFDMA transmission in which one transmission channel isdivided into a plurality of subchannels and each STA performssimultaneous UL transmission on an allocated subchannel. Similarly, DLMU-MIMO transmission may be taken as an example of DL MU transmission inthe following examples of the present disclosure. However, the examplesof the present disclosure are also applicable in the same manner to DLOFDMA transmission in which one transmission channel is divided into aplurality of subchannels and simultaneous DL transmission is performedon respective subchannels allocated to STAs. That is, UL MU transmissionincludes UL MU-MIMO transmission or UL OFDMA transmission, and DL MUtransmission includes DL MU-MIMO transmission or DL OFDMA transmission,in the following description.

FIG. 22 depicts a sounding procedure including UL SU feedbacks accordingto the present disclosure.

In the example of FIG. 22, an AP corresponding to a beamformer transmitsan NDPA frame to STA1, STA2, STA3, and STA4 corresponding tobeamformees. Subsequently, the AP transmits an NDP frame to STA1, STA2,STA3, and STA4 after a predetermined IFS (e.g., an SIFS). STA1, STA2,STA3, and STA4 corresponding to beamformees may receive the NDPA frameand the NDP frame, determine that they are destination STAs of NDPsounding, and perform channel estimation using the NDP frame. The firstSTA (e.g., STA1) listed in an STA list included in the NDPA frame maytransmit a result of channel estimation in a HE Compressed Beamformingframe to the AP a predetermined IFS (e.g., an SIFS) after receiving theNDP frame. Each of STA2, STA3, and STA4 may transmit a result of channelestimation in a HE Compressed Beamforming frame to the AP apredetermined IFS (e.g., an SIFS) after receiving a Beamforming ReportPoll frame.

As described above, each of STA1, STA2, STA3, and STA4 may transmit afeedback frame (i.e., a HE Compressed Beamforming frame) in UL SUtransmission.

FIG. 23 depicts a sounding procedure including a UL MU feedbackaccording to the present disclosure.

If all of an AP corresponding to a beamformer and a plurality of STAscorresponding to beamformees support UL MU transmission, a procedure forreceiving HE Compressed Beamforming frames from the plurality of STAs bythe AP may be simplified.

FIG. 23 illustrates an example in which a plurality of STAs reportfeedback frames (e.g., UL MU HE Compressed Beamforming frames)simultaneously on a single channel in a WLAN system supporting UL MUtransmission (e.g., UL MU-MIMO transmission or UL OFDMA transmission).That is, a HE PPDU including the feedback frames from the plurality ofSTAs may be transmitted on the single channel in UL MU transmission.

The simultaneous transmissions of the UL MU HE Compressed Beamformingframe from the plurality of STAs may be configured or indicated by anNDPA frame or a Beamforming Report Poll frame destined for the pluralityof STAs in a sounding procedure. That is, the HE NDPA frame or theBeamforming Report Poll frame may further include information initiatingor eliciting UL MU transmission (e.g., UL MU transmission triggerinformation or UL MU transmission scheduling information). Or the HENDPA frame or the Beamforming Report Poll frame may include a triggerframe initiating or eliciting UL MU transmission.

For example, the beamformee STAs, STA1, STA2, STA3, and STA4 may receivean NDPA frame including UL MU transmission trigger information from thebeamformer, the AP and receive an NDP frame after a predetermined IFS(e.g., an SIFS). In this case, STA1, STA2, STA3, and STA4 maysimultaneously transmit HE PPDUs (i.e., UL MU HE Compressed Beamformingframes) including their channel estimation results in UL OFMDA or ULMU-MIMO a predetermined IFS (e.g., an SIFS) after receiving the NDPframe.

As described above, the NDPA frame or Beamforming Report Poll framecorresponding to a feedback trigger frame may include informationtriggering UL SU transmission in the example of FIG. 22 and informationtriggering UL MU transmission in the example of FIG. 23.

FIGS. 24 and 25 depict UL SU transmission of a feedback frame and UL MUtransmission of feedback frames in a sounding procedure according to thepresent disclosure.

It may not be assumed that all of STAs corresponding to beamformees(i.e., destination STAs of an NDPA frame and an NDP frame) support UL MUtransmission. Accordingly, the STAs corresponding to beamformees may bedivided into UL MU transmission supported STAs and UL MU transmissionnon-supported STAs, and a UL transmission scheme may be determinedaccording to the STA classification, for transmission of a frame inresponse to a feedback trigger frame (e.g., NDPA and NDP frames, or aBeamforming Report Poll frame).

The AP may request transmission of a UL MU transmission-based feedbackframe (i.e., a UL MU HE Compressed Beamforming frame) to UL MUtransmission supported STAs. That is, the AP may provide feedbacktrigger information indicating a UL MU transmission-based response typeto the UL MU transmission supported STAs. On the other hand, the AP mayrequest transmission of a UL SU transmission-based feedback frame (i.e.,a Legacy Compressed Beamforming frame, a UL SU HE Compressed Beamformingframe, or a simple HE Compressed Beamforming frame) to a UL MUtransmission non-supported STA(s). That is, the AP may provide feedbacktrigger information indicating a UL SU transmission-based response typeto the UL MU transmission non-supported STA(s).

Feedback frames of different types (i.e., a UL MU transmission type anda UL SU transmission type) may not be transmitted simultaneously.Therefore, a feedback trigger frame (e.g., NDPA and NDP frames, or aBeamforming Report Poll frame) may be provided such that feedback framesof different types may be transmitted at different time points. Forexample, to receive feedback frames (e.g., a UL MU HE CompressedBeamforming frame and a HE Compressed Beamforming frame) from aplurality of STAs including UL MU transmission supported STAs and a ULMU transmission non-supported STA(s), the AP may transmit informationtriggering UL MU transmission in an NDPA frame or a Beamforming ReportPoll frame sequentially to the UL MU transmission supported STAs and theUL MU transmission non-supported STA(s).

In the examples of FIGS. 24 and 25, it is assumed that STA1, STA2, andSTA3 are UL MU transmission supported STAs and STA4 is a UL MUtransmission non-supported STA. If the destination STAs of an NDPA frameand an NDP frame include UL MU transmission supported STAs and a UL MUtransmission non-supported STA, the NDPA frame may include informationtriggering UL transmission for one or more STAs supporting one (a firsttype) of different feedback frame types, and a Beamforming Report Pollframe may include information eliciting UL transmission for one or moreSTAs supporting the other feedback frame type (a second type). The firsttype may be a UL MU transmission-based feedback type, and the secondtype may be a UL SU transmission-based feedback type. Or the first typemay be the UL SU transmission-based feedback type, and the second typemay be the UL MU transmission-based feedback type.

Further, if the destination STAs of an NDPA frame and an NDP frameinclude UL MU transmission supported STAs and a UL MU transmissionnon-supported STA, the NDPA frame and the Beamforming Report Poll framecorresponding to feedback trigger frames may include informationindicating a PPDU type to be used as a feedback frame by the STAscorresponding to beamformees, that is, legacy PPDU (or UL SU PPDU) or ULMU PPDU.

In the example of FIG. 24, STA1, STA2, and STA3 supporting UL MUtransmission (i.e., supporting the UL MU transmission-based feedbacktype) are set in the first place (i.e., as the first three STAs) in aSTA Info field including a list of STAs corresponding to beamformees inthe NDPA frame. In this case, STA1, STA2, and STA3 may transmit UL MUtransmission-based feedback frames (i.e., UL MU HE CompressedBeamforming frames) to the AP a predetermined IFS (e.g., an SIFS) afterreceiving the NDPA frame and the NDP frame. That is, the UL MUtransmission supported STAs (i.e., the STAs supporting the UL MUtransmission-based response type), STA1, STA2, and STA3 may transmit ULMU HE Compressed Beamforming frames. Since different channel estimationsequences (e.g., different HE-STF and HE-LTF sequences) are used for theplurality of STAs participating in the UL MU transmission, the AP mayreceive the feedback frames transmitted simultaneously by the pluralityof STAs without collision.

In the example of FIG. 24, STA1, STA2, and STA3 supporting UL MUtransmission are set in the first place (i.e., as the first three STAs)and STA4 that does not support UL MU transmission (i.e., that supportsonly the UL SU transmission-based response type) is listed in thefollowing place, in the STA list included in the NDPA frame. In thiscase, STA4 may transmit a UL SU transmission-based CompressedBeamforming frame (a Legacy Compressed Beamforming frame, a UL SU HECompressed Beamforming frame, or a simple HE Compressed Beamformingframe) to the AP a predetermined IFS (e.g., an SIFS) after receiving theBeamforming Report Poll frame.

Meanwhile, in the example of FIG. 25, STA4 that does not support UL MUtransmission (i.e., that supports only the UL SU transmission-basedresponse type) is set in the first place and STA1, STA2, and STA3supporting UL MU transmission are listed in the following place, in theSTA list included in the NDPA frame. In this case, STA4 may transmit aUL SU transmission-based Compressed Beamforming frame (a LegacyCompressed Beamforming frame, a UL SU HE Compressed Beamforming frame,or a simple HE Compressed Beamforming frame) to the AP a predeterminedIFS (e.g., an SIFS) after receiving the NDPA frame and the NDP frame.STA1, STA2, and STA3 may transmit UL MU HE Compressed Beamforming framesto the AP a predetermined IFS (e.g., an SIFS) after receiving theBeamforming Report Poll frame from the AP. That is, the STAs supportingUL MU transmission (i.e., the STAs supporting the UL MUtransmission-based response type), STA1, STA2, and STA3 may transmitfeedback frames simultaneously in UL MU transmission. Since differentchannel estimation sequences (e.g., different HE-STF and HE-LTFsequences) are used for the plurality of STAs participating in the UL MUtransmission, the AP may receive the feedback frames transmittedsimultaneously by the plurality of STAs without collision.

As described above with reference to the examples of FIGS. 24 and 25,the PPDU type of a feedback frame elicited by a feedback trigger frame(an NDPA frame or a Beamforming Report Poll frame) may be the UL SUtransmission type (e.g., a legacy PPDU type or a UL SU PPDU type) or theUL MU transmission type (e.g., a UL MU PPDU type), and the type of thefeedback frame may be determined based on information (UL transmissiontriggering information) included in the feedback trigger frame (the NDPAframe or the Beamforming Report Poll frame).

If a feedback frame type for the first STA listed in the STA listincluded in the NDPA frame is set as the MU type, a STA set to the samefeedback frame type as the first STA, that is, the MU type from amongone or more STAs following the first STA may transmit a UL MU HECompressed Beamforming frame a predetermined IFS (e.g., an SIFS) afterreceiving the NDP frame. Or if the feedback frame type for the first STAlisted in the STA list included in the NDPA frame is set as the SU type,only the first STA may transmit a UL SU Compressed Beamforming frame apredetermined IFS (e.g., an SIFS) after receiving the NDP frame.

If an immediate response to the feedback trigger frame (e.g., the NDPAand NDP frames, or the Beamforming Report Poll frame) is of the UL MUtransmission type, identification information about different channelestimation sequences (e.g., different HE-STF and HE-LTF sequences) to beused for transmission of UL MU HE Compressed Beamforming frames by theplurality of STAs may be included in the NDPA frame or the BeamformingReport Poll frame corresponding to a feedback trigger frame. Theidentification information about the channel estimation sequences may bedefined as information indicating an element from a set of a pluralityof elements corresponding to a plurality of channel estimation sequences(or scrambling codes with which to generate the channel estimationsequences). That is, the identification information about the channelestimation sequences corresponds to information that allocatesdistinguishable resources (e.g., sequence resources or code resources)to the plurality of STAs for UL MU transmission of the plurality ofSTAs.

Also, the channel estimation sequence identification information for ULMU transmission may be included as a UL MU Sequence ID field in the NDPAframe or Beamforming Report Poll frame corresponding to a feedbacktrigger frame. The channel estimation sequence identificationinformation for UL MU transmission (e.g., the UL MU Sequence ID field)may be included in the STA Info field of the NDPA frame or BeamformingReport Poll frame, or in a VHT Control field, a HE Control field, aService field, etc. Further, the channel estimation sequenceidentification information for UL MU transmission may be provided asSTA-specific control information for each STA.

If the channel estimation sequence identification information for UL MUtransmission (e.g., the UL MU Sequence ID field) has a predeterminedvalue (e.g., 0), it may indicate that a feedback frame transmitted apredetermined IFS (e.g., the SIFS) after reception of the feedbacktrigger frame (e.g., the NDPA and NDP frames, or the Beamforming ReportPoll frame) is of the UL MU PPDU type. In this case, one channelestimation sequence (or a scrambling code with which to generate thechannel estimation sequence) from the set of the plurality of channelestimation sequences (or the scrambling codes with which to generate thechannel estimation sequences) may be determined based on the value ofthe channel estimation sequence identification information for UL MUtransmission (e.g., the UL MU Sequence ID field).

Referring to FIG. 24 again, STA1, STA2, and STA3 supporting UL MUtransmission (i.e., the UL MU transmission-based response type) are setin the first place in the STA list included in the NDPA frame and thevalues of UL MU Sequence ID fields are set to 1, 2, and 3 respectivelyfor STA1, STA2, and STA3. When STA1, STA2, and STA3 transmit the UL MUHE Compressed Beamforming frames to the AP the predetermined IFS (e.g.,the SIFS) after receiving the NDPA frame and the NDP frame, STA1, STA2,and STA3 may perform the UL MU transmission using different channelestimation sequences corresponding to the values of the UL MU SequenceID fields, 1, 2, and 3. STA4 that does not support UL MU transmission(i.e., that supports only the UL SU transmission-based response type) isset in the following place in the STA list included in the NDPA frameand the value of a UL MU Sequence ID field is set to 0 for STA4. In thiscase, STA4 may transmit the UL SU transmission-based CompressedBeamforming frame (the Legacy Compressed Beamforming frame, the UL SU HECompressed Beamforming frame, or the simple HE Compressed Beamformingframe) to the AP the predetermined IFS (e.g., the SIFS) after receivingthe Beamforming Report Poll frame from the AP.

Referring to FIG. 25 again, STA4 that does not support UL MUtransmission (i.e., that supports only the UL SU transmission-basedresponse type) is set in the first place in the STA list included in theNDPA frame and the value of a UL MU Sequence ID field is set to 0 forSTA4. STA4 may transmit the UL SU transmission-based CompressedBeamforming frame (the Legacy Compressed Beamforming frame, the UL SU HECompressed Beamforming frame, or the simple HE Compressed Beamformingframe) to the AP the predetermined IFS (e.g., the SIFS) after receivingthe NDPA frame and the NDP frame from the AP. Meanwhile, STA1, STA2, andSTA3 supporting UL MU transmission (i.e., the UL MU transmission-basedresponse type) are set in the following place in the STA list includedin the NDPA frame and the values of UL MU Sequence ID fields are set to1, 2, and 3 respectively for STA1, STA2, and STA3. When STA1, STA2, andSTA3 transmit the UL MU HE Compressed Beamforming frames to the AP thepredetermined IFS (e.g., the SIFS) after receiving the BeamformingReport Poll frame, STA1, STA2, and STA3 may perform the UL MUtransmission using different channel estimation sequences correspondingto the values of the UL MU Sequence ID fields, 1, 2, and 3.

Like the afore-described NDPA frame, the Beamforming Report Poll frametransmitted to the plurality of STAs (e.g., STA1, STA2, and STA3) by theAP may include information indicating the type (e.g., the UL MUtransmission-based PPDU type) of a UL PPDU corresponding to an immediateresponse to the Beamforming Report Poll frame and channel estimationsequence identification information (e.g., a UL MU Sequence ID subfield)for UL MU transmission of each participating STA.

Additionally, if STAs supporting UL MU transmission (i.e., STAssupporting the UL MU transmission-based response type) transmit feedbackframes simultaneously in a UL MU PPDU to the AP, the transmission timesof the feedback frames transmitted by the plurality of STAs may coincidewith each other. If the transmission times of the feedback framestransmitted by the plurality of STAs are different, the overhead ofprocessing the feedback frames in the AP increases. To prevent theincrease of the overhead, the transmission times of the feedback framestransmitted by the plurality of STAs may be made aligned with each other(e.g., the feedback frames transmitted by the plurality of STAs may bemade start at the same time and end at the same time). If thetransmission times of the feedback frames transmitted by the pluralityof STAs coincide with each other, this may mean that the feedback framestransmitted by the plurality of STAs have the same MCS.

In the example of FIG. 24, the NDPA frame may include MCS informationfor UL MU transmission so that the feedback frames transmitted by STA1,STA2, and STA3 supporting UL MU transmission (i.e., the UL MUtransmission-based response type) may coincide with each other. That is,the plurality of STAs participating in the UL MU transmission may use anMCS value indicated by the MCS information for UL MU transmission,included in the NDPA frame, in transmission of a UL MU PPDU includingthe feedback frames. Or the MCS information for UL MU transmission(e.g., a UL MU MCS field) may be included in the STA Info field of theNDPA frame or the Beamforming Report Poll frame, or in the VHT Controlfield, the HE Control field, the Service field, etc. Further, the MCSinformation for UL MU transmission may be provided as STA-specificcontrol information for each STA, or common control information for theSTAs. Or an MCS applied to a UL MU HE Compressed Beamforming frame maybe determined based on the MCS value of the NDPA frame.

In the example of FIG. 25, like the NDPA frame illustrated in FIG. 24,the Beamforming Report Poll frame may include MCS information for UL MUtransmission so that the transmission times of the feedback framestransmitted by STA1, STA2, and STA3 supporting UL MU transmission (i.e.,the UL MU transmission-based response type) may coincide with eachother. Or an MCS applied to the UL MU HE Compressed Beamforming framemay be determined based on an MCS value of the Beamforming Report Pollframe.

That is, the MCS applied to the UL MU HE Compressed Beamforming framemay be determined implicitly based on the MCS value of the NDPA frame orthe Beamforming Report Poll frame corresponding to a feedback triggerframe, or information indicating the MCS applied to the UL MU HECompressed Beamforming frame may be included explicitly in the NDPAframe or the Beamforming Report Poll frame corresponding to a feedbacktrigger frame.

As described before, a frame eliciting UL MU transmission may includeinformation based on which one or more of the type of a UL MU PPDU,resources to be used in the UL MU transmission, a UL MU transmissiontime, and an MCS of the UL MU transmission are determined That is, theframe eliciting UL MU transmission may include one or more ofinformation based on which the type of a UL MU PPDU (i.e., the UL MUtransmission-based PPDU type or the UL SU transmission-based PPDU type)is determined, information based on which resources (e.g., sequenceresources or code resources) to be used in the UL MU transmission aredetermined, information based on which a UL MU transmission time isdetermined, and information based on which an MCS of the UL MUtransmission is determined.

In addition, when UL MU transmission supported STAs (i.e., STAssupporting the UL MU transmission-based response type) transmit feedbackframes simultaneously in a UL MU PPDU, Duration fields included in therespective feedback frames have the same value. The value of a Durationfield included in a UL MU HE Compressed Beamforming frame transmitted asan immediate response to an NDPA frame and an NDP frame may be set to avalue calculated by subtracting the transmission time of the NDP frame,the transmission time of the UL MU HE Compressed Beamforming frame, anda predetermined IFS (e.g., an SIFS) from the value of a Duration fieldincluded in the NDPA frame eliciting the UL MU HE Compressed Beamformingframe. Or the value of the Duration field included in the UL MU HECompressed Beamforming frame transmitted as an immediate response to theBeamforming Report Poll frame may be set to a value calculated bysubtracting the transmission time of the UL MU HE Compressed Beamformingframe and a predetermined IFS (e.g., an SIFS) from the value of aDuration field included in the Beamforming Report Poll frame elicitingthe UL MU HE Compressed Beamforming frame.

Now, a description will be given of examples of the present disclosureregarding a configuration of a feedback trigger frame.

NDP frames may be classified into two types. A type 1 NDP frame may bedefined as a frame from which an STA receiving the type 1 NDP frameacquires a channel state for OFDM beamforming transmission and DL/ULMU-MIMO-OFDM transmission. A type 2 NDP frame may be defined as a framefrom which an STA receiving the type 2 NDP frame acquires a channelstate for OFDMA beamforming transmission and DL/UL MU-MIMO-OFDMAtransmission. Accordingly, a type 1 Compressed Beamforming response mayinclude channel state information needed to calculate a beamformingsteering matrix based on a type 1 NDP, and a type 2 CompressedBeamforming response may include channel state information needed tocalculate a beamforming steering matrix based on a type 2 NDP. If thetypes of an NDP frame and a Compressed Beamforming response are not madeclear in the following description, the NDP frame may include a type 1NDP frame and a type 2 NDP frame, and the Compressed Beamformingresponse may include a type 1 Compressed Beamforming response and a type2 Compressed Beamforming response.

UL OFDMA transmission or UL MU-MIMO transmission may be applied toCompressed Beamforming responses (e.g., Compressed Beamforming frames)transmitted in response to a feedback trigger frame (e.g., NDPA and NDPframes, or a Beamforming Report Poll frame) in order to multiplexfeedback frames of a plurality of STAs. For UL OFDMA transmission,subchannel allocation information for the plurality of STAs may beincluded in the feedback trigger frame, and for UL MU-MIMO transmission,spatial stream allocation information for the plurality of STAs may beincluded in the feedback trigger frame. Further, code resources orsequence resources may be used to multiplex the plurality of STAs. Inthis case, code resource allocation information or sequence resourceallocation information (e.g., channel estimation sequence identificationinformation for UL MU transmission (e.g., a UL MU Sequence ID field), asdescribed in an example of the present disclosure) may be included inthe feedback trigger frame.

That is, the feedback trigger frame may include resource allocationinformation for UL MU transmission of the plurality of STAs. Theresource allocation information may include one or more of subchannelallocation information, spatial stream allocation information, codeallocation information, and sequence allocation information.

The resource allocation information may be included in anAggregated-MPDU (A-MPDU) or an MPDU of the feedback trigger frame. Forexample, an A-MPDU may be configured by combining a frame including theresource allocation information with an NDPA frame, and a PPDU includingthe A-MPDU may be transmitted to each beamformee. That is, an A-MPDUincluded in a DL MU PPDU (e.g., an NDPA frame or Beamforming Report Pollframe corresponding to a feedback trigger frame) directed to theplurality of STAs before the UL MU transmission (e.g., transmission ofCompressed Beamforming frames) of the plurality of STAs may include aframe carrying resource allocation information for each of the pluralityof STAs (i.e., a unicast trigger frame for one of the plurality ofSTAs).

Or the resource allocation information may be included in an MPDU (e.g.,a MAC header or MPDU payload) of the feedback trigger frame and a PPDUincluding this NDPA frame may be transmitted to the beamformees.

If resource allocation information is transmitted in an A-MPDU of afeedback trigger frame, up to 1 frame including the resource allocationinformation may be included in the A-MPDU. The frame including theresource allocation information may be the first or last MPDU of theA-MPDU.

If the feedback trigger frame is an NDPA frame, information indicatingwhether the first feedback report PPDU (e.g., a Compressed BeamformingFrame) for the NDPA frame and an NDP frame is an SU PPDU format or an MUPPDU format may be included in the NDPA frame.

If the first feedback report PPDU format indicated by the NDPA frame isthe MU PPDU format, an operation for transmitting a CompressedBeamforming response by a destination STA of the NDPA and NDP frames mayvary depending on whether the destination STA has received the MPDUincluding the resource allocation information after successfullyreceiving the NDPA frame. For example, if the first feedback report PPDUformat is indicated as the MU PPDU format, an STA receiving the MPDUincluding the resource allocation information may transmit a CompressedBeamforming frame using resources indicated by the received resourceallocation information, an SIFS or an NDP transmission time afterreceiving the NDPA frame. Meanwhile, an STA failing to receive the frameincluding the resource allocation information may not perform anyoperation except calculation of information to be included in aCompressed Beamforming Report, even though the first feedback reportPPDU format indicated by the NDPA frame is the MU PPDU format.

If the first feedback report PPDU format indicated by the NDPA frame isthe SU PPDU format, the first STA indicated by the STA Info field (i.e.,the first STA listed in the STA Info field) of the NDPA frame maytransmit a Compressed Beamforming frame in SU transmission, an SIFS andan NDP transmission time after receiving the NDPA frame.

If the feedback trigger frame is a Beamforming Report Poll frame,information indicating whether the format of a feedback report PPDU(e.g., a Compressed Beamforming Frame) for the Beamforming Report Pollframe is the SU PPDU format or the MU PPDU format may be included in theBeamforming Report Poll frame.

If the feedback report PPDU format indicated by the Beamforming ReportPoll frame is the MU PPDU format, an operation for transmitting aCompressed Beamforming response by an STA receiving the BeamformingReport Poll frame may vary depending on whether the STA has received anMPDU including resource allocation information after successfullyreceiving the Beamforming Report Poll frame. For example, if thefeedback report PPDU format is indicated as the MU PPDU format, an STAreceiving the MPDU including the resource allocation information maytransmit a Compressed Beamforming frame using resources indicated by thereceived resource allocation information, an SIFS after receiving theBeamforming Report Poll frame. Meanwhile, an STA failing to receive theframe including the resource allocation information may not perform anyoperation, even though the feedback report PPDU format indicated by theBeamforming Report Poll frame is the MU PPDU format.

If the feedback report PPDU format indicated by the Beamforming ReportPoll frame is the SU PPDU format, an STA indicated by the RA field ofthe Beamforming Report Poll frame may transmit a Compressed Beamformingframe in SU transmission, an SIFS after receiving the Beamforming ReportPoll frame.

FIG. 26 depicts an exemplary format of a Beamforming Report Poll frameaccording to the present disclosure.

The format illustrated in FIG. 26 may be used to indicate resourceallocation information in the Beamforming Report Poll frame.

An RA field may be set to a value corresponding to an address of an STArequested to transmit a Compressed Beamforming frame (i.e., adestination STA) in response to the Beamforming Report Poll frame. Ifthe Compressed Beamforming frame is transmitted in UL MU transmission,the RA field may be set to a broadcast address.

A TA field may be set to a value corresponding to an address of an STAtransmitting the Beamforming Report Poll frame.

The Beamforming Report Poll frame may include one or more BeamformingReport Poll STA Info fields. Beamforming Report Poll STA Info mayinclude information for multiplexing Compressed Beamforming Responses.For example, one Beamforming Report Poll STA Info field may include anAID subfield, a Feedback Report PPDU Format subfield, a Feedback SegmentRetransmission Bitmap subfield, and a Resource Allocation Informationsubfield.

The AID subfield may be set to a value corresponding to an AID of abeamformee (or 11 Least Significant Bits (LSBs) of the AID of thebeamformee) requested to transmit a Compressed Beamforming Report.

The Feedback Report PPDU Format subfield may be set to a valueindicating one of the SU PPDU format and the MU PPDU format. If abeamformee requests transmission of a Compressed Beamforming Report inMU transmission (e.g., in OFDMA), the beamformer may set the FeedbackReport PPDU Format subfield to a value corresponding to MU (e.g., 1). Orif a beamformee requests transmission of a Compressed Beamforming Reportin SU transmission, the beamformer may set the Feedback Report PPDUFormat subfield to a value corresponding to SU (e.g., 0).

The Feedback Segment Retransmission Bitmap subfield may be set to avalue indicating a feedback segment requested for the CompressedBeamforming Report. If an n^(th) bit position out of 8 bit positions ofa feedback segment retransmission bitmap is set to 1, this indicatesthat a feedback segment having a Remaining Feedback Segments subfield ofa (HE) MIMO Control field corresponding to n is requested (if n is 0,this means an LSB and if n is 7, this means a Most Significant Bit(MSB)). Or if the n^(th) bit position is set to 0, this indicates thatthe feedback segment having the Remaining Feedback Segments subfield ofthe (HE) MIMO Control field corresponding to n is not requested.

The Resource Allocation Information subfield may be set to a valuecorresponding to resources to be used for UL MU transmission of theCompressed Beamforming Report. The Resource Allocation Informationsubfield may include one or more of, for example, subchannel allocationinformation, spatial stream allocation information, code resourceallocation information, and sequence resource allocation information.That is, the Resource Allocation Information subfield may be set to avalue indicating a specific subchannel allocated to an STA, a valueindicating a specific spatial stream allocated to the STA, a valueindicating specific code resources allocated to the STA, or a valueindicating specific sequence resources allocated to the STA. Only if theFeedback Report PPDU Format subfield is set to a value corresponding toMU, the Resource Allocation Information subfield may be included in theBeamforming Report Poll STA Info field. If the Feedback Report PPDUFormat subfield is set to a value corresponding to SU, the ResourceAllocation Information subfield may not be included in the BeamformingReport Poll STA Info field.

FIG. 27 depicts an exemplary method according to the present disclosure.

In step S2710, a transmitting STA (e.g., an AP corresponding to abeamformer) may transmit a trigger frame to an STA group including oneor more responding STAs (e.g., STAs corresponding to beamformees). Thetrigger frame may include an NDPA frame or Beamforming Report Poll framecorresponding to a feedback trigger frame described in the forgoingexamples. The trigger frame may include information about transmissionof a response frame in response to the trigger frame. The response framemay include a (HE) Compressed Beamforming frame (i.e., a frame includinga beamforming report) corresponding to a feedback frame described in theforegoing examples. The information about transmission of a responseframe may include information about the type of a response frame (i.e.,the SU type or the MU type), information about an MCS of the responseframe, and resource allocation information for one or more STAstransmitting response frames.

In step S2720, each of responding STAs belonging to the STA group maydetermine the type of a response frame elicited by the trigger framebased on the information included in the received trigger frame (e.g.,the information about the type of a response frame). If the type of aresponse frame is determined to be the MU type, the responding STA maydetermine an MCA value to be applied to the response frame based on theMCS information included in the trigger frame in step S2730. Forexample, the MCS of the response frame may be determined implicitlybased on an MCS of the trigger frame or determined according to the MCSinformation about the response frame, included in the trigger frame. Instep S2740, the responding STA may determine resources (e.g., asubchannel, a resource unit, a spatial stream, a code, a sequence, etc.)to be used for transmission of the response frame based on the resourceallocation information included in the trigger frame. Accordingly, theplurality of responding STAs may transmit MU response framessimultaneously to the transmitting STA in step S2750.

If the type of the response frame is determined to be the SU type instep S2720, one responding STA may transmit an SU response frame to thetransmitting STA in step S2760.

FIG. 28 depicts another exemplary method according to the presentdisclosure.

FIG. 28 illustrates a specific example including the exemplary methodaccording to the present disclosure, illustrated in FIG. 27.

In step S2810, a beamformer STA (e.g., an AP) may transmit an NDPA frameto an STA group including one or more beamformee STAs (hereinafter,referred to as a beamformee STA group). The NDPA frame may includeinformation about a type of a first feedback report, MCS informationabout the first feedback report, and resource allocation information forthe first feedback report. The information about the first feedbackreport may indicate the UL MU transmission type or the UL SUtransmission type. Step S2810 of FIG. 28 may correspond to step S2710 ofFIG. 27.

In step S2813, the beamformer STA may transmit an NDP frame to thebeamformee STA group.

In step S2815, one or more STAs (e.g., one or more STAs to whichresources for UL transmission have been allocated by the resourceallocation information included in the NDPA frame) may generate firstfeedback reports based on the information included in the NDPA frame(i.e., the type, MCS, and resource allocation information of the firstfeedback reports). Specifically, the one or more STAs may determine thetype of the first feedback reports and generate the first feedbackreports in the UL SU type or the UL MU type. Step 2815 of FIG. 28 maycorrespond to step S2720 of FIG. 27 or steps S2720, S2730, and S2740 ofFIG. 27.

In step S2817, the one or more STAs may transmit the first feedbackreports (e.g., Compressed Beamforming frames) to the beamformer STA.Step S2817 of FIG. 28 may correspond to step S2750 or S2760 of FIG. 27.

If there is any remaining STA except for the STAs that have transmittedthe first feedback reports in the beamformer STA group, the beamformerSTA may transmit a Beamforming Report Poll frame to one or more of theremaining STAs in step S2820. The Beamforming Report Poll frame mayinclude information about the type of feedback reports to be transmittedby the one or more STAs, MCS information about the feedback reports, andresource allocation information for the feedback reports. Theinformation about the feedback reports may indicate one of the UL MUtransmission type and the UL SU transmission type. Step S2820 of FIG. 28may correspond to step S2710 of FIG. 27.

In step S2823, the one or more STAs may generate the feedback reportsbased on the information included in the Beamforming Report Poll framereceived in step S2820 (i.e., the type, MCS, and resource allocationinformation of the feedback reports). Specifically, the one or more STAsdetermine the type of the feedback reports and generate the feedbackreports in the UL SU type or the UL MU type. Step 2823 of FIG. 28 maycorrespond to step S2720 of FIG. 27 or steps S2720, S2730, and S2740 ofFIG. 27.

In step S2825, the one or more STAs may transmit the feedback reports(e.g., Compressed Beamforming frames) to the beamformer STA. Step S2825of FIG. 28 may correspond to step S2750 or S2760 of FIG. 27.

Subsequently, if there is any remaining STA except for the STAs thathave transmitted the first feedback reports (S2817) and the STAs thathave transmitted the additional feedback reports (S2825) in thebeamformer STA group, a step for transmitting a Beamforming Report Pollframe to one or more of the remaining STAs by the beamformer STA, andthus generating feedback reports and transmitting the feedback reportsto the beamformer STA by the one or more STAs may further be performed.This corresponds to performing the afore-described steps S2820, S2823,and S2825 for the one or more additional STAs. This step may beadditionally performed until all STAs of the beamformee STA grouptransmit feedback reports to the beamformer STA. Finally, the beamformerSTA may transmit a Beamforming Report Poll frame in step S2830, and thusone or more STAs generate feedback reports in step S2833 and transmitthe feedback reports in step S2835. Thus, the exemplary NDP soundingprocedure according to the present disclosure may be completed.

While the afore-described exemplary methods of present disclosure havebeen described as a series of operations for simplicity of description,this does not limit the sequence of steps. When needed, steps may beperformed at the same time or in a different sequence. All of theexemplary steps are not always necessary to implement the methodproposed by the present disclosure.

The foregoing embodiments of the present disclosure may be implementedseparately or combinations of two or more of the embodiments may beimplemented simultaneously, for the afore-described exemplary methods ofpresent disclosure.

The present disclosure includes an apparatus for processing orperforming the method of the present disclosure (e.g., the wirelessdevice and its components described with reference to FIGS. 1, 2, and3).

The present disclosure includes software (an operating system (OS), anapplication, firmware, a program, etc.) for executing the method of thepresent disclosure in a device or a computer, and a medium storing thesoftware that can be executed in a device or a computer.

While various embodiments of the present disclosure have been describedin the context of an IEEE 802.11 system, they are applicable to variousmobile communication systems.

What is claimed is:
 1. A method for transmitting a response frame by aresponding station (STA) to a transmitting STA in a wireless local areanetwork, the method comprising: receiving, from the transmitting STA, atrigger frame including information related to a Modulation and CodingScheme (MCS) for the response frame; and transmitting, to thetransmitting STA, the response frame according to an MCS determinedbased on the information related to the MCS for the response frameincluded in the trigger frame, wherein, when a type of the responseframe corresponds to a Multiple-User (MU) type, a same MCS is applied tothe response frame by a plurality of STAs including the STA and at leastone other STA.
 2. The method according to claim 1, wherein theinformation related to the MCS for the response frame is equal to an MCSof the trigger frame.
 3. The method according to claim 1, wherein theinformation related to the MCS for the response frame is provided ascommon information for the plurality of STAs.
 4. The method according toclaim 1, wherein the information related to the MCS for the responseframe is provided as STA-specific information for each of the pluralityof STAs.
 5. The method according to claim 1, wherein the trigger framefurther includes information related to a type of the response frame,the type of the response frame including a single-user (SU) type and theMU type.
 6. The method according to claim 5, wherein the responding STAtransmits the response frame having a type determined based on theinformation related to the type of the response frame included in thetrigger frame.
 7. The method according to claim 5, wherein, when thetype of the response frame corresponds to the MU type, the responseframe is simultaneously transmitted by the plurality of STAs.
 8. Themethod according to claim 1, wherein the trigger frame further includesresource allocation information for each of the plurality of STAs. 9.The method according to claim 8, wherein the resource allocationinformation includes information indicating distinguished resources forthe plurality of STAs.
 10. The method according to claim 8, wherein theresource allocation information includes subchannel allocationinformation for each of the plurality of STAs.
 11. The method accordingto claim 8, wherein the resource allocation information is included inan Aggregated-Medium Access Control Protocol Data Unit (A-MPDU) of thetrigger frame.
 12. The method according to claim 8, wherein the resourceallocation information is included in an MPDU of the trigger frame. 13.The method according to claim 5, wherein, when the type of the responseframe corresponds to the SU type, the response frame is transmitted onlyby the responding STA.
 14. The method according to claim 1, wherein thetrigger frame includes a Beamforming Report Poll frame, and the responseframe includes a Compressed Beamforming frame, wherein the CompressedBeamforming frame is transmitted as an immediate response to theBeamforming Report Poll frame.
 15. The method according to claim 1,wherein the trigger frame includes a Null Data Packet Announcement(NDPA) frame, and the response frame includes a Compressed Beamformingframe, wherein the Compressed Beamforming frame is transmitted by theresponding STA, after the responding STA receives the NDPA frame and aNull Data Packet (NDP) frame.
 16. The method according to claim 15,wherein the plurality of STAs corresponding to first plurality of STAInformation fields included in the NDPA frame, simultaneously transmitsthe Compressed Beamforming frame.
 17. A method for receiving a responseframe by a transmitting station (STA) from a responding STA in awireless local area network, the method comprising: transmitting, to theresponding STA, a trigger frame including information related to aModulation and Coding Scheme (MCS) for the response frame; andreceiving, from the responding STA, the response frame according to anMCS determined based on the information related to the MCS for theresponse frame included in the trigger frame, wherein, when a type ofthe response frame corresponds to a Multiple-User (MU) type, a same MCSis applied to the response frame by a plurality of STAs including theSTA and at least one other STA.